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/ui/mod.rs b/src/ui/mod.rs
index b071430..546f3e6 100644
--- a/src/ui/mod.rs
+++ b/src/ui/mod.rs
@@ -1,4 +1,5 @@
 mod style;
+pub mod preset;
 
 use bevy::prelude::*;
 use bevy_egui::egui;
diff --git a/src/ui/preset.rs b/src/ui/preset.rs
new file mode 100644
index 0000000..93a1290
--- /dev/null
+++ b/src/ui/preset.rs
@@ -0,0 +1,103 @@
+//! 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)]
+pub enum Preset {
+    Cinematic,
+    Performance,
+    Web,
+    /// Read-only marker set when any preset-touched field is hand-edited
+    /// away from a preset bundle. `apply(Custom, _)` is a no-op.
+    Custom,
+}
+
+/// 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 (it never
+/// matches any real params state, by construction — see `hashed_fields`).
+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.as_u32_().hash(&mut hasher);
+        self.disk_quality.as_u32().hash(&mut hasher);
+        self.aa_quality.samples().hash(&mut hasher);
+        hasher.finish()
+    }
+}
+
+// Local trait adapters: BloomQuality has levels(), DiskQuality has as_u32(),
+// AaQuality has samples(). Unify under one name for hashing.
+trait AsU32ForHash { fn as_u32_(self) -> u32; }
+impl AsU32ForHash for BloomQuality { fn as_u32_(self) -> u32 { self.levels() } }
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));
+}