diff --git a/src/lib.rs b/src/lib.rs
index 55ada65..872a60d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1 +1,12 @@
+// Lib crate root. The bin (main.rs) consumes these via `singularity_rs::...`
+// rather than redeclaring the modules itself; otherwise both crate roots
+// would try to own the same file-level modules (compile error: E0428/
+// E0152). This is why every module the binary needs lives here.
+pub mod camera;
+pub mod params;
 pub mod physics;
+pub mod render;
+pub mod scene;
+pub mod ui;
+#[cfg(target_arch = "wasm32")]
+pub mod web;
diff --git a/src/main.rs b/src/main.rs
index 71c0841..499151d 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,14 +1,12 @@
 use bevy::prelude::*;
 use bevy::window::WindowPlugin;
 
+// The bin consumes the lib crate's modules; it does NOT redeclare them.
+// src/lib.rs owns every file-level module (camera/params/physics/render/
+// scene/ui/web), so we import what main() needs from the lib root.
+use singularity_rs::render::BlackHolePlugin;
 #[cfg(target_arch = "wasm32")]
-mod web;
-mod render;
-mod camera;
-mod params;
-mod ui;
-mod scene;
-mod physics;
+use singularity_rs::web;
 
 fn main() {
     // On web, abort startup if WebGPU isn't available and show a message.
@@ -50,6 +48,6 @@ fn main() {
                 // Dropping it removes that noise and shrinks the wasm binary.
                 .disable::<bevy::audio::AudioPlugin>(),
         )
-        .add_plugins(render::BlackHolePlugin)
+        .add_plugins(BlackHolePlugin)
         .run();
 }
diff --git a/src/render/plugin.rs b/src/render/plugin.rs
index 1938029..453c685 100644
--- a/src/render/plugin.rs
+++ b/src/render/plugin.rs
@@ -131,7 +131,12 @@ impl Plugin for BlackHolePlugin {
             .add_systems(Update, rebuild_bloom)
             // bevy_egui 0.41 requires UI systems to run inside the egui context
             // pass (fonts/ctx are initialized there); placing them in Update panics.
-            .add_systems(bevy_egui::EguiPrimaryContextPass, crate::ui::ui_system);
+            // setup_egui_style is a one-shot (Local<bool> guard): retries until the
+            // context exists, applies the theme once, never runs again.
+            .add_systems(
+                bevy_egui::EguiPrimaryContextPass,
+                (crate::ui::setup_egui_style, crate::ui::ui_system).chain(),
+            );
     }
 }
 
diff --git a/src/ui.rs b/src/ui.rs
deleted file mode 100644
index 83cf5f7..0000000
--- a/src/ui.rs
+++ /dev/null
@@ -1,185 +0,0 @@
-use bevy::prelude::*;
-use bevy_egui::egui;
-
-pub fn ui_system(
-    mut contexts: bevy_egui::EguiContexts,
-    mut params: ResMut<crate::params::BlackHoleParams>,
-    mut camera: ResMut<crate::camera::OrbitCamera>,
-    mut wants: ResMut<crate::camera::WantsPointer>,
-    mut planet_dirty: ResMut<crate::scene::planets::PlanetSystemDirty>,
-) {
-    if let Ok(ctx) = contexts.ctx_mut() {
-        egui::Window::new("Controls")
-            .collapsible(true)
-            .resizable(true)
-            .default_pos([16.0, 16.0])
-            .default_width(300.0)
-            .default_height(560.0)
-            .show(ctx, |ui| {
-            egui::ScrollArea::vertical().show(ui, |ui| {
-                egui::CollapsingHeader::new("Camera")
-                    .default_open(true)
-                    .show(ui, |ui| {
-                        ui.add(egui::Slider::new(&mut camera.distance, 3.0..=200.0).text("Distance"));
-                        ui.add(egui::Slider::new(&mut camera.yaw, -std::f32::consts::PI..=std::f32::consts::PI).text("Yaw"));
-                        ui.add(egui::Slider::new(&mut camera.pitch, (-std::f32::consts::PI + 0.05)..=(std::f32::consts::PI - 0.05)).text("Pitch"));
-                        ui.add(egui::Slider::new(&mut camera.fov, 0.3..=2.0).text("FOV"));
-                    });
-                egui::CollapsingHeader::new("Black Hole")
-                    .default_open(true)
-                    .show(ui, |ui| {
-                        ui.add(egui::Slider::new(&mut params.spin, 0.0..=1.0).text("Spin (χ)"));
-                        ui.label(format!("ISCO (disk inner): {:.3}", crate::physics::kerr_isco(params.spin)));
-                        ui.label(format!("Horizon r+: {:.3}", crate::physics::kerr_horizon(params.spin)));
-                    });
-                egui::CollapsingHeader::new("Accretion Disk")
-                    .default_open(true)
-                    .show(ui, |ui| {
-                        // disk_inner removed — now spin-derived (see Black Hole section).
-                        ui.add(egui::Slider::new(&mut params.disk_outer, 6.0..=50.0).text("Outer radius"));
-                        ui.add(egui::Slider::new(&mut params.disk_tilt, 0.0..=std::f32::consts::PI).text("Tilt"));
-                        ui.add(egui::Slider::new(&mut params.disk_brightness, 0.0..=3.0).text("Brightness"));
-                        ui.add(egui::Slider::new(&mut params.disk_rotation_speed, 0.0..=3.0).text("Rotation speed"));
-                        use crate::params::DiskColorMode;
-                        let mut cm = params.disk_color_mode;
-                        egui::ComboBox::from_label("Color model")
-                            .selected_text(format!("{:?}", cm))
-                            .show_ui(ui, |ui| {
-                                ui.selectable_value(&mut cm, DiskColorMode::Gradient, "Gradient");
-                                ui.selectable_value(&mut cm, DiskColorMode::Blackbody, "Blackbody");
-                            });
-                        params.disk_color_mode = cm;
-                        ui.add_enabled(
-                            cm == DiskColorMode::Blackbody,
-                            egui::Slider::new(&mut params.disk_temp, 1000.0..=50000.0).text("Temperature (K)"),
-                        );
-                    });
-                egui::CollapsingHeader::new("Planets")
-                    .default_open(false)
-                    .show(ui, |ui| {
-                        // Record state before edits; if seed/count/k/enabled change,
-                        // flag dirty so spawn_planet_system regenerates next frame.
-                        // (time_scale excluded: it only scales orbit_system's time,
-                        //  no respawn needed.)
-                        let prev = (
-                            params.planets_enabled,
-                            params.planet_count_target,
-                            params.planet_radius_factor,
-                            params.planet_seed,
-                        );
-                        ui.checkbox(&mut params.planets_enabled, "Enable");
-                        ui.add(egui::Slider::new(&mut params.planet_count_target, 0..=8).text("Count"));
-                        ui.add(egui::Slider::new(&mut params.planet_radius_factor, 1.1..=2.0).text("Radius (× disk outer)"));
-                        ui.label(format!("Orbit r = {:.2} (disk outer: {:.1})", params.planet_radius_factor * params.disk_outer, params.disk_outer));
-                        ui.add(egui::Slider::new(&mut params.planet_seed, 0..=1000).text("Seed"));
-                        ui.add(egui::Slider::new(&mut params.planet_time_scale, 1.0..=200.0).text("Time scale"));
-                        let curr = (
-                            params.planets_enabled,
-                            params.planet_count_target,
-                            params.planet_radius_factor,
-                            params.planet_seed,
-                        );
-                        if curr != prev {
-                            planet_dirty.0 = true;
-                        }
-                    });
-                egui::CollapsingHeader::new("Disk Turbulence")
-                    .default_open(true)
-                    .show(ui, |ui| {
-                        use crate::params::DiskQuality;
-                        let mut q = params.disk_quality;
-                        egui::ComboBox::from_label("Disk quality")
-                            .selected_text(format!("{:?}", q))
-                            .show_ui(ui, |ui| {
-                                ui.selectable_value(&mut q, DiskQuality::Off, "Off");
-                                ui.selectable_value(&mut q, DiskQuality::Low, "Low");
-                                ui.selectable_value(&mut q, DiskQuality::Medium, "Medium");
-                                ui.selectable_value(&mut q, DiskQuality::High, "High");
-                            });
-                        params.disk_quality = q;
-                        let on = q != DiskQuality::Off;
-                        ui.add_enabled(on, egui::Slider::new(&mut params.disk_half_thickness, 0.02..=0.3).text("Thickness (H/R)"));
-                        ui.add_enabled(on, egui::Slider::new(&mut params.filament_freq, 0.2..=4.0).text("Filament frequency"));
-                        ui.add_enabled(on, egui::Slider::new(&mut params.filament_sharpness, 1.0..=6.0).text("Filament sharpness"));
-                        ui.add_enabled(on, egui::Slider::new(&mut params.density_freq, 0.2..=3.0).text("Density frequency"));
-                        ui.add_enabled(on, egui::Slider::new(&mut params.density_strength, 0.0..=2.0).text("Density strength"));
-                        ui.add_enabled(on, egui::Slider::new(&mut params.arm_count, 0.0..=6.0).text("Arm count"));
-                        ui.add_enabled(on, egui::Slider::new(&mut params.arm_tightness, 0.0..=6.0).text("Arm tightness"));
-                        ui.add_enabled(on, egui::Slider::new(&mut params.arm_strength, 0.0..=1.0).text("Arm strength"));
-                    });
-                egui::CollapsingHeader::new("Doppler").show(ui, |ui| {
-                    ui.checkbox(&mut params.doppler_enabled, "Enabled");
-                    ui.add_enabled(params.doppler_enabled, egui::Slider::new(&mut params.doppler_strength, 0.0..=3.0).text("Strength"));
-                });
-                egui::CollapsingHeader::new("Jets").show(ui, |ui| {
-                    ui.checkbox(&mut params.jets_enabled, "Enabled");
-                    // Mirror the shader's spin gate (sample_jets in black_hole.wgsl):
-                    // jets are a spin-powered (Blandford-Znajek) outflow and render
-                    // only for χ ≥ 0.05. When the user enables them at low spin,
-                    // explain the no-op so the checkbox doesn't look broken.
-                    let jets_renderable = params.spin >= 0.05;
-                    if params.jets_enabled && !jets_renderable {
-                        ui.label("Spin (χ) too low — jets need χ ≥ 0.05.");
-                    }
-                    ui.add_enabled(
-                        params.jets_enabled && jets_renderable,
-                        egui::Slider::new(&mut params.jets_strength, 0.0..=3.0).text("Strength"),
-                    );
-                });
-                egui::CollapsingHeader::new("Renderer").show(ui, |ui| {
-                    ui.add(egui::Slider::new(&mut params.steps, 50..=600).text("Steps"));
-                    ui.add(egui::Slider::new(&mut params.render_scale, 0.25..=1.0).text("Render scale"));
-                });
-                egui::CollapsingHeader::new("Background").show(ui, |ui| {
-                    ui.add(egui::Slider::new(&mut params.star_intensity, 0.0..=3.0).text("Star intensity"));
-                    ui.add(egui::Slider::new(&mut params.skybox_intensity, 0.0..=3.0).text("Skybox intensity"));
-                });
-                egui::CollapsingHeader::new("Grid").show(ui, |ui| {
-                    ui.checkbox(&mut params.grid_enabled, "Enabled");
-                    ui.add_enabled(params.grid_enabled, egui::Slider::new(&mut params.grid_density, 0.1..=4.0).text("Density"));
-                });
-                egui::CollapsingHeader::new("Quality")
-                    .default_open(true)
-                    .show(ui, |ui| {
-                        use crate::params::BloomQuality;
-                        let mut q = params.bloom_quality;
-                        egui::ComboBox::from_label("Bloom quality")
-                            .selected_text(format!("{:?}", q))
-                            .show_ui(ui, |ui| {
-                                ui.selectable_value(&mut q, BloomQuality::Off, "Off");
-                                ui.selectable_value(&mut q, BloomQuality::Low, "Low");
-                                ui.selectable_value(&mut q, BloomQuality::Medium, "Medium");
-                                ui.selectable_value(&mut q, BloomQuality::High, "High");
-                            });
-                        params.bloom_quality = q;
-                        ui.add_enabled(q != BloomQuality::Off, egui::Slider::new(&mut params.bloom_threshold, 0.0..=3.0).text("Bloom threshold"));
-                        ui.add_enabled(q != BloomQuality::Off, egui::Slider::new(&mut params.bloom_strength, 0.0..=2.0).text("Bloom strength"));
-                        ui.add(egui::Slider::new(&mut params.exposure, 0.5..=3.0).text("Exposure"));
-                        ui.add(egui::Slider::new(&mut params.render_scale, 0.25..=1.0).text("Resolution scale"));
-                        ui.checkbox(&mut params.star_aa, "Anti-aliased stars");
-                        {
-                            // Per-pixel supersampling: antialiases the higher-order
-                            // lensed-image rings on the disk into a smooth gradient.
-                            // Cost scales linearly with sample count (each sub-ray
-                            // runs the full RK45 march).
-                            use crate::params::AaQuality;
-                            let mut a = params.aa_quality;
-                            egui::ComboBox::from_label("Ring anti-alias")
-                                .selected_text(format!("{:?} ({}×)", a, a.samples()))
-                                .show_ui(ui, |ui| {
-                                    ui.selectable_value(&mut a, AaQuality::Off, "Off (1×)");
-                                    ui.selectable_value(&mut a, AaQuality::Low, "Low (2×)");
-                                    ui.selectable_value(&mut a, AaQuality::High, "High (4×)");
-                                });
-                            params.aa_quality = a;
-                        }
-                        ui.label("MSAA is decorative on a fullscreen shader (no geometry edges to sample).");
-                    });
-            });
-            });
-        // egui captures pointer when the cursor is over a window or being interacted with.
-        wants.0 = ctx.egui_wants_pointer_input();
-    } else {
-        wants.0 = false;
-    }
-}
diff --git a/src/ui/mod.rs b/src/ui/mod.rs
new file mode 100644
index 0000000..04dda25
--- /dev/null
+++ b/src/ui/mod.rs
@@ -0,0 +1,259 @@
+mod sections;
+mod style;
+pub mod preset;
+
+use bevy::prelude::*;
+use bevy_egui::egui;
+use egui::{LayerId, UiBuilder};
+
+use crate::ui::preset::{Preset, apply, canonical_hash, params_hash};
+use crate::ui::style::ACCENT_CYAN;
+
+/// One-shot egui styling. Registered in `EguiPrimaryContextPass` with a
+/// `Local<bool>` guard so it retries until `ctx_mut()` first succeeds, then
+/// runs exactly once. Per-frame set_style/set_visuals would dirty layout
+/// caches every frame; this avoids that.
+pub fn setup_egui_style(
+    mut contexts: bevy_egui::EguiContexts,
+    mut done: Local<bool>,
+) {
+    if *done {
+        return;
+    }
+    if let Ok(ctx) = contexts.ctx_mut() {
+        crate::ui::style::setup(&ctx);
+        *done = true;
+    }
+}
+
+/// An always-open framed card. Title in cyan `RichText::strong().small()`.
+/// Used for the 4 most-tuned groups (Camera / Black Hole / Disk / Quality).
+fn group(ui: &mut egui::Ui, title: &str, body: impl FnOnce(&mut egui::Ui)) {
+    egui::Frame::group(ui.style())
+        .inner_margin(8.0)
+        .corner_radius(6.0)
+        .stroke(egui::Stroke::new(
+            1.0,
+            ui.visuals().window_stroke.color,
+        ))
+        .show(ui, |ui| {
+            ui.set_width(ui.available_width());
+            ui.label(
+                egui::RichText::new(title)
+                    .strong()
+                    .small()
+                    .color(ACCENT_CYAN),
+            );
+            ui.add_space(2.0);
+            body(ui);
+        });
+}
+
+/// A collapsible section (`default_open` controls initial state).
+/// Used for the 6 secondary groups. The body closure is only invoked when open.
+fn collapsing(
+    ui: &mut egui::Ui,
+    id: &str,
+    title: &str,
+    default_open: bool,
+    body: impl FnOnce(&mut egui::Ui),
+) {
+    egui::CollapsingHeader::new(title)
+        .default_open(default_open)
+        .id_salt(id)
+        .show(ui, |ui| {
+            ui.set_width(ui.available_width());
+            body(ui);
+        });
+}
+
+/// Collapsible section whose header hosts an enable toggle on the right.
+/// Used by Doppler / Jets / Grid / Planets (design §4.1). The body closure
+/// receives the current `enabled` state so it can `add_enabled` its rows.
+///
+/// State persistence is handled by `HeaderResponse::body`, which calls
+/// `CollapsingState::store` internally via `show_body_indented` (see egui
+/// 0.35 `containers/collapsing_header.rs`). `show_header` consumes the
+/// state, so there is no separate `store` call here.
+fn collapsing_with_toggle(
+    ui: &mut egui::Ui,
+    id: &str,
+    title: &str,
+    default_open: bool,
+    enabled: &mut bool,
+    body: impl FnOnce(&mut egui::Ui, bool),
+) {
+    let id = ui.make_persistent_id(id);
+    let state =
+        egui::collapsing_header::CollapsingState::load_with_default_open(ui.ctx(), id, default_open);
+    state
+        .show_header(ui, |ui| {
+            ui.checkbox(enabled, title);
+        })
+        .body(|ui| {
+            ui.set_width(ui.available_width());
+            body(ui, *enabled);
+        });
+}
+
+/// The top preset bar. `current` is the UI-layer state (which preset is
+/// shown as selected); `just_applied` is set for one frame after a preset
+/// is chosen, to skip the Custom-detection hash compare on that frame
+/// (applying a preset changes params; that change must not flip to Custom).
+fn preset_bar(
+    ui: &mut egui::Ui,
+    params: &mut crate::params::BlackHoleParams,
+    current: &mut Preset,
+    just_applied: &mut bool,
+) {
+    ui.horizontal(|ui| {
+        ui.label("Preset:");
+        let prev = *current;
+        egui::ComboBox::from_id_salt("preset_combo")
+            .selected_text(format!("{:?}", prev))
+            .show_ui(ui, |ui| {
+                ui.selectable_value(current, Preset::Cinematic, "Cinematic");
+                ui.selectable_value(current, Preset::Performance, "Performance");
+                ui.selectable_value(current, Preset::Web, "Web");
+                ui.selectable_value(current, Preset::Custom, "Custom");
+            });
+        if *current != prev && *current != Preset::Custom {
+            // User picked a concrete preset → apply its bundle.
+            apply(*current, params);
+            *just_applied = true;
+        }
+        ui.with_layout(egui::Layout::right_to_left(egui::Align::Center), |ui| {
+            // Global reset-to-default for all params.
+            if ui.button("↺ all").clicked() {
+                *params = crate::params::BlackHoleParams::default();
+                *current = Preset::Custom;
+                *just_applied = true;
+            }
+        });
+    });
+    ui.separator();
+}
+
+pub fn ui_system(
+    mut contexts: bevy_egui::EguiContexts,
+    mut params: ResMut<crate::params::BlackHoleParams>,
+    mut camera: ResMut<crate::camera::OrbitCamera>,
+    mut wants: ResMut<crate::camera::WantsPointer>,
+    mut planet_dirty: ResMut<crate::scene::planets::PlanetSystemDirty>,
+    mut current_preset: Local<Preset>,
+    mut just_applied: Local<bool>,
+    mut last_hash: Local<Option<u64>>,
+) {
+    // Local<Preset> defaults to Custom (Preset::default()).
+    // Local<Option<u64>> defaults to None — first-frame sentinel.
+
+    let Ok(ctx) = contexts.ctx_mut() else {
+        wants.0 = false;
+        return;
+    };
+
+    // --- Custom-detection (skip on the frame a preset was just applied) ---
+    let now_hash = params_hash(&params);
+    if !*just_applied && last_hash.is_some() && now_hash != last_hash.unwrap() {
+        // Some preset-touched field changed by hand. If it no longer matches
+        // the current concrete preset's canonical bundle, flip to Custom.
+        let matches_any = matches!(
+            *current_preset,
+            Preset::Cinematic | Preset::Performance | Preset::Web
+        ) && canonical_hash(*current_preset) == now_hash;
+        if !matches_any && *current_preset != Preset::Custom {
+            *current_preset = Preset::Custom;
+        }
+    }
+    *just_applied = false;
+    *last_hash = Some(now_hash);
+
+    // --- Chassis ---
+    // egui 0.35's Panel::show needs a parent `&mut Ui`, but bevy_egui's
+    // single-pass `EguiPrimaryContextPass` only hands us a `Context`. Create
+    // the top-level Ui manually (matches bevy_egui 0.41's side_panel example).
+    // `Context` is an `Arc`, so cloning it here is cheap.
+    let mut root_ui = egui::Ui::new(
+        ctx.clone(),
+        "controls_root".into(),
+        UiBuilder::new()
+            .layer_id(LayerId::background())
+            .max_rect(ctx.viewport_rect()),
+    );
+    egui::Panel::right("controls")
+        .default_size(300.0)
+        .size_range(260.0..=400.0)
+        .resizable(true)
+        .show(&mut root_ui, |ui| {
+            // Top bar (fixed).
+            preset_bar(ui, &mut params, &mut current_preset, &mut just_applied);
+
+            // Section stack (scrolls).
+            egui::ScrollArea::vertical().show(ui, |ui| {
+                use crate::ui::sections::*;
+
+                // --- Always-open cards ---
+                group(ui, "Camera", |ui| section_camera(ui, &mut camera));
+                group(ui, "Black Hole", |ui| section_black_hole(ui, &mut params));
+                group(ui, "Accretion Disk", |ui| section_disk(ui, &mut params));
+                group(ui, "Quality", |ui| section_quality(ui, &mut params));
+
+                // --- Collapsing sections ---
+                collapsing(ui, "turbulence", "Disk Turbulence", false,
+                    |ui| section_turbulence(ui, &mut params));
+
+                // collapsing_with_toggle takes `&mut bool` for the header
+                // checkbox AND a body closure that mutates `params`. Passing
+                // `&mut params.<field>` plus a closure borrowing `&mut params`
+                // is a double-mutable-borrow; copy the field out, pass a local
+                // ref, then write it back. (All four are `bool` → Copy.)
+                let mut en = params.doppler_enabled;
+                collapsing_with_toggle(ui, "doppler", "Doppler", false,
+                    &mut en,
+                    |ui, en| section_doppler(ui, &mut params, en));
+                params.doppler_enabled = en;
+
+                let mut en = params.jets_enabled;
+                collapsing_with_toggle(ui, "jets", "Jets", false,
+                    &mut en,
+                    |ui, en| section_jets(ui, &mut params, en));
+                params.jets_enabled = en;
+
+                // Planets: snapshot dirty-relevant fields before rendering so
+                // we can detect changes (relocated from the old Planets block).
+                let prev_planet = (
+                    params.planets_enabled,
+                    params.planet_count_target,
+                    params.planet_radius_factor,
+                    params.planet_seed,
+                );
+                let mut en = params.planets_enabled;
+                collapsing_with_toggle(ui, "planets", "Planets", false,
+                    &mut en,
+                    |ui, en| section_planets(ui, &mut params, en));
+                params.planets_enabled = en;
+                let curr_planet = (
+                    params.planets_enabled,
+                    params.planet_count_target,
+                    params.planet_radius_factor,
+                    params.planet_seed,
+                );
+                if curr_planet != prev_planet {
+                    planet_dirty.0 = true;
+                }
+
+                collapsing(ui, "background", "Background", false,
+                    |ui| section_background(ui, &mut params));
+
+                let mut en = params.grid_enabled;
+                collapsing_with_toggle(ui, "grid", "Grid", false,
+                    &mut en,
+                    |ui, en| section_grid(ui, &mut params, en));
+                params.grid_enabled = en;
+            });
+        });
+
+    // egui captures pointer when the cursor is over a window or being
+    // interacted with. MUST stay last — load-bearing for orbit camera.
+    wants.0 = ctx.egui_wants_pointer_input();
+}
diff --git a/src/ui/preset.rs b/src/ui/preset.rs
new file mode 100644
index 0000000..7f954a5
--- /dev/null
+++ b/src/ui/preset.rs
@@ -0,0 +1,101 @@
+//! Preset bundles + Custom-edit detection (design spec §3).
+//!
+//! `params_hash` hashes ONLY the fields that presets touch — otherwise a
+//! non-preset edit (e.g. disk_tilt) would spuriously flip the bar to Custom.
+//! The field set below is the single source of truth; if you add a field to
+//! a preset bundle, add it to `hashed_fields` too or the test
+//! `non_preset_field_change_flips_to_custom` will not catch the leak.
+
+use crate::params::{AaQuality, BlackHoleParams, BloomQuality, DiskQuality};
+
+use std::hash::{Hash, Hasher};
+
+#[derive(Clone, Copy, PartialEq, Eq, Debug, Default)]
+pub enum Preset {
+    #[default]
+    /// Read-only marker set when any preset-touched field is hand-edited
+    /// away from a preset bundle. `apply(Custom, _)` is a no-op.
+    Custom,
+    Cinematic,
+    Performance,
+    Web,
+}
+
+/// The exact params each preset writes. `Custom` writes nothing.
+fn bundle(p: Preset) -> Option<HashedParams> {
+    // Defaults chosen to mirror cfg!(wasm32) dual-defaults already in the codebase:
+    // Cinematic = desktop default, Web = wasm default, Performance = a low tier.
+    Some(match p {
+        Preset::Cinematic => HashedParams {
+            steps: 300, render_scale: 0.75,
+            bloom_quality: BloomQuality::High, disk_quality: DiskQuality::High, aa_quality: AaQuality::High,
+        },
+        Preset::Performance => HashedParams {
+            steps: 150, render_scale: 0.5,
+            bloom_quality: BloomQuality::Low, disk_quality: DiskQuality::Low, aa_quality: AaQuality::Off,
+        },
+        Preset::Web => HashedParams {
+            steps: 200, render_scale: 0.5,
+            bloom_quality: BloomQuality::Low, disk_quality: DiskQuality::Low, aa_quality: AaQuality::Off,
+        },
+        Preset::Custom => return None,
+    })
+}
+
+/// Apply a preset's bundle to params. `Custom` is a no-op.
+pub fn apply(p: Preset, params: &mut BlackHoleParams) {
+    if let Some(b) = bundle(p) {
+        params.steps = b.steps;
+        params.render_scale = b.render_scale;
+        params.bloom_quality = b.bloom_quality;
+        params.disk_quality = b.disk_quality;
+        params.aa_quality = b.aa_quality;
+    }
+}
+
+/// Stable hash of a preset's canonical bundle. `Custom` returns 0.
+/// (The Custom case is never fed to a `canonical_hash == params_hash`
+/// comparison: `ui_system`'s preset-detection arm only checks
+/// Cinematic | Performance | Web, so the 0 sentinel never risks matching.)
+pub fn canonical_hash(p: Preset) -> u64 {
+    match bundle(p) {
+        Some(b) => b.hash(),
+        None => 0,
+    }
+}
+
+/// Hash of the preset-touched fields of a live params. Used by `ui_system`
+/// to detect hand-edits and flip the bar to Custom.
+pub fn params_hash(params: &BlackHoleParams) -> u64 {
+    let h = HashedParams {
+        steps: params.steps,
+        render_scale: params.render_scale,
+        bloom_quality: params.bloom_quality,
+        disk_quality: params.disk_quality,
+        aa_quality: params.aa_quality,
+    };
+    h.hash()
+}
+
+// --- internals ---
+
+#[derive(Clone, Copy)]
+struct HashedParams {
+    steps: u32,
+    render_scale: f32,
+    bloom_quality: BloomQuality,
+    disk_quality: DiskQuality,
+    aa_quality: AaQuality,
+}
+
+impl HashedParams {
+    fn hash(&self) -> u64 {
+        let mut hasher = std::collections::hash_map::DefaultHasher::new();
+        self.steps.hash(&mut hasher);
+        self.render_scale.to_bits().hash(&mut hasher); // f32: hash bit pattern, not value
+        self.bloom_quality.levels().hash(&mut hasher);
+        self.disk_quality.as_u32().hash(&mut hasher);
+        self.aa_quality.samples().hash(&mut hasher);
+        hasher.finish()
+    }
+}
diff --git a/src/ui/sections.rs b/src/ui/sections.rs
new file mode 100644
index 0000000..ed0e209
--- /dev/null
+++ b/src/ui/sections.rs
@@ -0,0 +1,253 @@
+//! Per-group section render functions. Each takes the `&mut egui::Ui` it
+//! draws into plus the params/camera refs it needs. The `ui_system`
+//! orchestrator (mod.rs) calls them inside `group()` / `collapsing()`.
+
+use bevy_egui::egui;
+
+use crate::camera::OrbitCamera;
+use crate::params::{
+    AaQuality, BlackHoleParams, BloomQuality, DiskColorMode,
+};
+use crate::ui::style::{ACCENT_ORANGE, MUTED_TEXT};
+
+use std::f32::consts::PI;
+
+/// Shared two-column row helper: label + sized widget.
+fn row(
+    ui: &mut egui::Ui,
+    label: &str,
+    add_widget: impl FnOnce(&mut egui::Ui) -> egui::Response,
+) {
+    ui.label(label);
+    add_widget(ui);
+    ui.end_row();
+}
+
+// ============================ Always-open cards ============================
+
+pub fn section_camera(ui: &mut egui::Ui, cam: &mut OrbitCamera) {
+    egui::Grid::new("camera_grid").num_columns(2).spacing([8.0, 4.0])
+        .show(ui, |ui| {
+            row(ui, "Distance", |ui| ui.add_sized([140.0, 16.0],
+                egui::Slider::new(&mut cam.distance, 3.0..=200.0)
+                    .suffix(" r_g").fixed_decimals(1)));
+            row(ui, "Yaw", |ui| ui.add_sized([140.0, 16.0],
+                egui::Slider::new(&mut cam.yaw, -PI..=PI)
+                    .suffix(" rad").fixed_decimals(2)));
+            row(ui, "Pitch", |ui| ui.add_sized([140.0, 16.0],
+                egui::Slider::new(&mut cam.pitch, (-PI + 0.05)..=(PI - 0.05))
+                    .suffix(" rad").fixed_decimals(2)));
+            row(ui, "FOV", |ui| ui.add_sized([140.0, 16.0],
+                egui::Slider::new(&mut cam.fov, 0.3..=2.0).fixed_decimals(2)));
+        });
+}
+
+pub fn section_black_hole(ui: &mut egui::Ui, params: &mut BlackHoleParams) {
+    egui::Grid::new("bh_grid").num_columns(2).spacing([8.0, 4.0])
+        .show(ui, |ui| {
+            row(ui, "Spin (χ)", |ui| ui.add_sized([140.0, 16.0],
+                egui::Slider::new(&mut params.spin, 0.0..=1.0).fixed_decimals(2)));
+            // Read-only derived values: right column, muted color.
+            ui.label(egui::RichText::new("ISCO (disk inner)").color(MUTED_TEXT));
+            ui.label(egui::RichText::new(format!("{:.3}", crate::physics::kerr_isco(params.spin)))
+                .color(MUTED_TEXT));
+            ui.end_row();
+            ui.label(egui::RichText::new("Horizon r+").color(MUTED_TEXT));
+            ui.label(egui::RichText::new(format!("{:.3}", crate::physics::kerr_horizon(params.spin)))
+                .color(MUTED_TEXT));
+            ui.end_row();
+        });
+}
+
+pub fn section_quality(ui: &mut egui::Ui, params: &mut BlackHoleParams) {
+    // Bloom quality combobox + threshold/strength (hidden when Off — §4.3).
+    {
+        let mut q = params.bloom_quality;
+        egui::Grid::new("bloom_grid").num_columns(2).spacing([8.0, 4.0])
+            .show(ui, |ui| {
+                ui.label("Bloom");
+                egui::ComboBox::from_id_salt("bloom_combo")
+                    .selected_text(format!("{:?}", q))
+                    .show_ui(ui, |ui| {
+                        ui.selectable_value(&mut q, BloomQuality::Off, "Off");
+                        ui.selectable_value(&mut q, BloomQuality::Low, "Low");
+                        ui.selectable_value(&mut q, BloomQuality::Medium, "Medium");
+                        ui.selectable_value(&mut q, BloomQuality::High, "High");
+                    });
+                ui.end_row();
+            });
+        params.bloom_quality = q;
+    }
+    if params.bloom_quality != BloomQuality::Off {
+        egui::Grid::new("bloom_detail_grid").num_columns(2).spacing([8.0, 4.0])
+            .show(ui, |ui| {
+                row(ui, "Threshold", |ui| ui.add_sized([140.0, 16.0],
+                    egui::Slider::new(&mut params.bloom_threshold, 0.0..=3.0).fixed_decimals(2)));
+                row(ui, "Strength", |ui| ui.add_sized([140.0, 16.0],
+                    egui::Slider::new(&mut params.bloom_strength, 0.0..=2.0).fixed_decimals(2)));
+            });
+    }
+    egui::Grid::new("renderer_grid").num_columns(2).spacing([8.0, 4.0])
+        .show(ui, |ui| {
+            // steps + render_scale migrated here from the deleted Renderer section.
+            row(ui, "Steps", |ui| ui.add_sized([140.0, 16.0],
+                egui::Slider::new(&mut params.steps, 50..=600)
+                    .logarithmic(true).fixed_decimals(0)));
+            row(ui, "Resolution", |ui| ui.add_sized([140.0, 16.0],
+                egui::Slider::new(&mut params.render_scale, 0.25..=1.0)
+                    .logarithmic(true).fixed_decimals(2)));
+            row(ui, "Exposure", |ui| ui.add_sized([140.0, 16.0],
+                egui::Slider::new(&mut params.exposure, 0.5..=3.0).fixed_decimals(2)));
+            row(ui, "Star AA", |ui| ui.checkbox(&mut params.star_aa, ""));
+            // Ring anti-alias (supersampling for lensed-image rings).
+            let mut a = params.aa_quality;
+            ui.label("Ring AA");
+            egui::ComboBox::from_id_salt("aa_combo")
+                .selected_text(format!("{:?} ({}×)", a, a.samples()))
+                .show_ui(ui, |ui| {
+                    ui.selectable_value(&mut a, AaQuality::Off, "Off (1×)");
+                    ui.selectable_value(&mut a, AaQuality::Low, "Low (2×)");
+                    ui.selectable_value(&mut a, AaQuality::High, "High (4×)");
+                });
+            ui.end_row();
+            params.aa_quality = a;
+        });
+    ui.label(
+        egui::RichText::new("MSAA is decorative on a fullscreen shader (no geometry edges).")
+            .small().color(MUTED_TEXT),
+    );
+}
+
+pub fn section_disk(ui: &mut egui::Ui, params: &mut BlackHoleParams) {
+    egui::Grid::new("disk_grid").num_columns(2).spacing([8.0, 4.0])
+        .show(ui, |ui| {
+            row(ui, "Outer radius", |ui| ui.add_sized([140.0, 16.0],
+                egui::Slider::new(&mut params.disk_outer, 6.0..=50.0)
+                    .suffix(" r_g").fixed_decimals(1)));
+            row(ui, "Tilt", |ui| ui.add_sized([140.0, 16.0],
+                egui::Slider::new(&mut params.disk_tilt, 0.0..=PI)
+                    .suffix(" rad").fixed_decimals(2)));
+            row(ui, "Brightness", |ui| ui.add_sized([140.0, 16.0],
+                egui::Slider::new(&mut params.disk_brightness, 0.0..=3.0).fixed_decimals(2)));
+            row(ui, "Rotation", |ui| ui.add_sized([140.0, 16.0],
+                egui::Slider::new(&mut params.disk_rotation_speed, 0.0..=3.0).fixed_decimals(2)));
+            // Color model combobox.
+            let mut cm = params.disk_color_mode;
+            ui.label("Color model");
+            egui::ComboBox::from_id_salt("disk_color_combo")
+                .selected_text(format!("{:?}", cm))
+                .show_ui(ui, |ui| {
+                    ui.selectable_value(&mut cm, DiskColorMode::Gradient, "Gradient");
+                    ui.selectable_value(&mut cm, DiskColorMode::Blackbody, "Blackbody");
+                });
+            ui.end_row();
+            params.disk_color_mode = cm;
+        });
+    // Temperature only meaningful in Blackbody mode — disable (not hide) so
+    // the user sees their value while briefly in Gradient.
+    let blackbody = params.disk_color_mode == DiskColorMode::Blackbody;
+    ui.add_enabled(
+        blackbody,
+        egui::Slider::new(&mut params.disk_temp, 1000.0..=50000.0)
+            .suffix(" K")
+            .logarithmic(true)
+            .fixed_decimals(0)
+            .text("Temperature"),
+    );
+}
+
+// ============================ Collapsing sections ==========================
+
+pub fn section_turbulence(ui: &mut egui::Ui, params: &mut BlackHoleParams) {
+    use crate::params::DiskQuality;
+    let mut q = params.disk_quality;
+    egui::Grid::new("diskq_grid").num_columns(2).spacing([8.0, 4.0])
+        .show(ui, |ui| {
+            ui.label("Disk quality");
+            egui::ComboBox::from_id_salt("diskq_combo")
+                .selected_text(format!("{:?}", q))
+                .show_ui(ui, |ui| {
+                    ui.selectable_value(&mut q, DiskQuality::Off, "Off");
+                    ui.selectable_value(&mut q, DiskQuality::Low, "Low");
+                    ui.selectable_value(&mut q, DiskQuality::Medium, "Medium");
+                    ui.selectable_value(&mut q, DiskQuality::High, "High");
+                });
+            ui.end_row();
+        });
+    params.disk_quality = q;
+
+    let on = q != DiskQuality::Off;
+    if !on {
+        ui.label(egui::RichText::new(
+            "Disk quality Off → flat zero-thickness disk rendered.",
+        ).small().color(ACCENT_ORANGE));
+    }
+    egui::Grid::new("turb_grid").num_columns(2).spacing([8.0, 4.0])
+        .show(ui, |ui| {
+            ui.add_enabled(on, egui::Slider::new(&mut params.disk_half_thickness, 0.02..=0.3).text("Thickness (H/R)"));
+            ui.end_row();
+            ui.add_enabled(on, egui::Slider::new(&mut params.filament_freq, 0.2..=4.0).text("Filament frequency"));
+            ui.end_row();
+            ui.add_enabled(on, egui::Slider::new(&mut params.filament_sharpness, 1.0..=6.0).text("Filament sharpness"));
+            ui.end_row();
+            ui.add_enabled(on, egui::Slider::new(&mut params.density_freq, 0.2..=3.0).text("Density frequency"));
+            ui.end_row();
+            ui.add_enabled(on, egui::Slider::new(&mut params.density_strength, 0.0..=2.0).text("Density strength"));
+            ui.end_row();
+            ui.add_enabled(on, egui::Slider::new(&mut params.arm_count, 0.0..=6.0).text("Arm count"));
+            ui.end_row();
+            ui.add_enabled(on, egui::Slider::new(&mut params.arm_tightness, 0.0..=6.0).text("Arm tightness"));
+            ui.end_row();
+            ui.add_enabled(on, egui::Slider::new(&mut params.arm_strength, 0.0..=1.0).text("Arm strength"));
+            ui.end_row();
+        });
+}
+
+pub fn section_doppler(ui: &mut egui::Ui, params: &mut BlackHoleParams, enabled: bool) {
+    ui.add_enabled(enabled, egui::Slider::new(&mut params.doppler_strength, 0.0..=3.0).text("Strength"));
+}
+
+pub fn section_jets(ui: &mut egui::Ui, params: &mut BlackHoleParams, enabled: bool) {
+    // Mirror the shader's spin gate: jets render only for χ ≥ 0.05.
+    let jets_renderable = params.spin >= 0.05;
+    if enabled && !jets_renderable {
+        ui.label(egui::RichText::new(
+            "Jets need χ ≥ 0.05 (Blandford-Znajek is spin-powered).",
+        ).small().color(ACCENT_ORANGE));
+    }
+    ui.add_enabled(
+        enabled && jets_renderable,
+        egui::Slider::new(&mut params.jets_strength, 0.0..=3.0).text("Strength"),
+    );
+}
+
+pub fn section_planets(ui: &mut egui::Ui, params: &mut BlackHoleParams, enabled: bool) {
+    ui.add_enabled(enabled, egui::Slider::new(&mut params.planet_count_target, 0..=8).text("Count"));
+    ui.add_enabled(enabled, egui::Slider::new(&mut params.planet_radius_factor, 1.1..=2.0).prefix("× ").text("Radius (× disk outer)"));
+    ui.add_enabled(
+        enabled,
+        egui::Label::new(
+            egui::RichText::new(format!(
+                "Orbit r = {:.2} (disk outer: {:.1})",
+                params.planet_radius_factor * params.disk_outer,
+                params.disk_outer
+            )).color(MUTED_TEXT),
+        ),
+    );
+    ui.add_enabled(enabled, egui::Slider::new(&mut params.planet_seed, 0..=1000).text("Seed"));
+    ui.add_enabled(enabled, egui::Slider::new(&mut params.planet_time_scale, 1.0..=200.0).text("Time scale"));
+}
+
+pub fn section_background(ui: &mut egui::Ui, params: &mut BlackHoleParams) {
+    egui::Grid::new("bg_grid").num_columns(2).spacing([8.0, 4.0])
+        .show(ui, |ui| {
+            row(ui, "Star intensity", |ui| ui.add_sized([140.0, 16.0],
+                egui::Slider::new(&mut params.star_intensity, 0.0..=3.0).fixed_decimals(2)));
+            row(ui, "Skybox", |ui| ui.add_sized([140.0, 16.0],
+                egui::Slider::new(&mut params.skybox_intensity, 0.0..=3.0).fixed_decimals(2)));
+        });
+}
+
+pub fn section_grid(ui: &mut egui::Ui, params: &mut BlackHoleParams, enabled: bool) {
+    ui.add_enabled(enabled, egui::Slider::new(&mut params.grid_density, 0.1..=4.0).text("Density"));
+}
diff --git a/src/ui/style.rs b/src/ui/style.rs
new file mode 100644
index 0000000..99a36aa
--- /dev/null
+++ b/src/ui/style.rs
@@ -0,0 +1,83 @@
+//! egui styling for the control panel. Deep-space cyan/orange theme,
+//! hand-applied over `Visuals::dark()`. See design spec §1.
+//!
+//! Pinned to egui 0.35: the `Shadow` struct packs fields into 8-bit integers,
+//! and the context style API is `set_theme` + `global_style_mut` (not the
+//! `ctx.style()`/`set_style()` of 0.34). See inline comments at each site.
+
+use bevy_egui::egui::{self, Color32, Stroke, TextStyle, Visuals};
+
+// --- Palette (single source of truth; referenced by every section helper) ---
+pub const ACCENT_CYAN: Color32 = Color32::from_rgb(90, 200, 255);   // #5AC8FF
+pub const ACCENT_ORANGE: Color32 = Color32::from_rgb(255, 140, 66); // #FF8C42
+pub const PANEL_FILL: Color32 = Color32::from_rgb(14, 16, 20);      // #0E1014
+pub const EXTREME_BG: Color32 = Color32::from_rgb(8, 9, 12);        // #08090C
+pub const MUTED_TEXT: Color32 = Color32::from_rgb(140, 140, 140);   // read-only / disabled labels
+
+/// Text sizes applied over egui's default `Proportional` family. No custom
+/// font binary is embedded (spec non-goal).
+pub fn text_styles() -> Vec<(TextStyle, f32)> {
+    vec![
+        (TextStyle::Body, 14.0),
+        (TextStyle::Monospace, 12.0),
+        (TextStyle::Heading, 16.0),
+        (TextStyle::Small, 11.0),
+    ]
+}
+
+/// The themed `Visuals`. Built fresh from `dark()` each call so it is
+/// independent of whatever egui's defaults evolve into.
+///
+/// Note: `animation_time` lives on `Style`, not `Visuals`, since egui 0.35;
+/// it is applied in [`setup`].
+pub fn sci_fi_visuals() -> Visuals {
+    let mut v = Visuals::dark();
+    v.panel_fill = PANEL_FILL;
+    v.extreme_bg_color = EXTREME_BG;
+    v.hyperlink_color = ACCENT_CYAN; // also used as the section-heading color by convention
+    v.selection.bg_fill = ACCENT_CYAN;
+    v.selection.stroke = Stroke::new(1.0, ACCENT_ORANGE);
+
+    v.widgets.inactive.weak_bg_fill = Color32::from_rgb(30, 36, 48); // #1E2430
+    v.widgets.inactive.bg_stroke = Stroke::new(0.5, Color32::from_rgb(60, 70, 70)); // #3C4646
+    v.widgets.hovered.weak_bg_fill = Color32::from_rgb(40, 50, 70); // #283246
+    v.widgets.hovered.fg_stroke = Stroke::new(1.0, Color32::from_rgb(200, 220, 255)); // #C8DCFF
+    v.widgets.active.fg_stroke = Stroke::new(1.0, ACCENT_CYAN);
+    v.widgets.active.weak_bg_fill = Color32::from_rgb(50, 70, 100); // #324664
+    v.widgets.noninteractive.bg_stroke = Stroke::new(0.5, Color32::from_rgb(40, 46, 60)); // #282E3C
+
+    // egui 0.35's `epaint::Shadow` packs its fields into 8-bit integers
+    // (`offset: [i8; 2]`, `blur: u8`, `spread: u8`) to keep the struct at 8
+    // bytes. Use integer literals; the f32 form would not type-check.
+    v.window_shadow = egui::epaint::Shadow {
+        offset: [0, 4],
+        blur: 16,
+        spread: 0,
+        color: Color32::from_black_alpha(120),
+    };
+    v
+}
+
+/// Apply fonts + spacing + visuals to a context. Called exactly once from
+/// `setup_egui_style` (plugin.rs).
+///
+/// egui 0.35 replaced `ctx.style()` / `ctx.set_style()` with theme-scoped
+/// accessors. We pin the active theme to Dark and mutate the active `Style`
+/// in place; `Visuals` and `animation_time` are both fields of `Style`, so
+/// they go through the same closure. `text_styles` is now a public
+/// `BTreeMap` field (no `text_styles_mut()`).
+pub fn setup(ctx: &egui::Context) {
+    ctx.set_theme(egui::Theme::Dark);
+    ctx.global_style_mut(|style| {
+        style.spacing.item_spacing = egui::vec2(8.0, 6.0);
+        style.spacing.slider_width = 150.0;
+        style.spacing.indent = 18.0;
+        style.spacing.button_padding = egui::vec2(10.0, 4.0);
+        style.spacing.indent_ends_with_horizontal_line = false;
+        style.animation_time = 0.12;
+        for (ts, size) in text_styles() {
+            style.text_styles.insert(ts, egui::FontId::proportional(size));
+        }
+        style.visuals = sci_fi_visuals();
+    });
+}
diff --git a/tests/preset_test.rs b/tests/preset_test.rs
new file mode 100644
index 0000000..b514ff0
--- /dev/null
+++ b/tests/preset_test.rs
@@ -0,0 +1,41 @@
+use singularity_rs::params::BlackHoleParams;
+use singularity_rs::ui::preset::{Preset, apply, canonical_hash, params_hash};
+
+#[test]
+fn canonical_hash_matches_just_applied_params() {
+    // After applying a preset, params_hash of the result must equal that
+    // preset's canonical_hash — otherwise the Custom-detection logic would
+    // immediately flip a freshly-applied preset back to Custom.
+    let mut p = BlackHoleParams::default();
+    for preset in [Preset::Cinematic, Preset::Performance, Preset::Web] {
+        apply(preset, &mut p);
+        assert_eq!(
+            canonical_hash(preset),
+            params_hash(&p),
+            "preset {:?}: apply() did not reproduce canonical_hash",
+            preset
+        );
+    }
+}
+
+#[test]
+fn non_preset_field_change_flips_to_custom() {
+    // Editing a field that NO preset touches (disk_tilt is not in any preset bundle)
+    // must NOT change params_hash. This guards the "hash only preset fields"
+    // invariant: non-preset edits must not spuriously flip to Custom.
+    let mut p = BlackHoleParams::default();
+    let h0 = params_hash(&p);
+    p.disk_tilt = 1.0; // not in any preset bundle
+    assert_eq!(h0, params_hash(&p), "non-preset field leaked into hash");
+}
+
+#[test]
+fn preset_field_change_differs_from_canonical() {
+    // Editing a preset-touched field after applying must change the hash
+    // away from the preset's canonical_hash (i.e. flip to Custom).
+    let mut p = BlackHoleParams::default();
+    apply(Preset::Cinematic, &mut p);
+    let h_canonical = canonical_hash(Preset::Cinematic);
+    p.steps = 299; // off by one from the Cinematic bundle
+    assert_ne!(h_canonical, params_hash(&p));
+}