fix(render): resolve validation crash from egui camera + brightpass uniform

Two runtime validation errors crashed the app on startup:

1. BrightPassMaterial used #[uniform(0)] threshold: f32 (bare f32 = 4-byte
   binding), but the WGSL BrightPassUniform struct is 16 bytes (threshold +
   3 pads). The pipeline-layout mismatch ("Buffer structure size 16...
   greater than min_binding_size, which is 4") killed the opaque_mesh2d
   pipeline. Fix: wrap threshold in a BrightPassUniform struct matching the
   WGSL layout, same pattern as BlurUniform/CompositeUniform.

2. bevy_egui 0.41's auto-context assigns PrimaryEguiContext to the FIRST
   Added<Camera> — with the 7-camera bloom pipeline, that's the offscreen
   camera (renders to Rgba16Float Image). egui's pipeline expects the
   window's Rgba8UnormSrgb surface → format mismatch crash. Fix: disable
   auto_create_primary_context (PreStartup system) and explicitly tag the
   composite (window) camera with PrimaryEguiContext.

Both fixes verified: the app now runs without validation errors for 8+
seconds. Physics tests (20/20) unaffected.

src/render/material.rs

@@ -123,12 +123,24 @@ impl Material2d for BlackHoleMaterial {
     }
 }
 
+/// Uniform for the bright-pass (bloom stage [2]).
+/// Must be a struct (not bare f32) so the Rust binding size matches the
+/// WGSL `BrightPassUniform` (16 bytes: threshold + 3 pads). A bare f32
+/// registers a 4-byte min_binding_size, causing a pipeline-layout mismatch.
+#[derive(Clone, ShaderType, Default)]
+pub struct BrightPassUniform {
+    pub threshold: f32,
+    pub _pad0: f32,
+    pub _pad1: f32,
+    pub _pad2: f32,
+}
+
 /// Extracts luminance above a threshold from the HDR offscreen into a
 /// half-res float texture (bloom stage [2]). Soft-knee, not hard cut.
 #[derive(Asset, TypePath, AsBindGroup, Clone)]
 pub struct BrightPassMaterial {
     #[uniform(0)]
-    pub threshold: f32,
+    pub uniform: BrightPassUniform,
     #[texture(1)]
     #[sampler(2)]
     pub source: Handle<Image>,

src/render/plugin.rs

@@ -80,6 +80,15 @@ pub struct BloomFinalTarget(pub Handle<Image>);
 #[derive(Resource)]
 pub struct AppliedBloomQuality(pub crate::params::BloomQuality);
 
+/// Disables bevy_egui's auto-context creation so we can explicitly assign
+/// `PrimaryEguiContext` to the composite (window) camera. Without this,
+/// bevy_egui assigns to the first spawned camera — the offscreen camera,
+/// which renders to an Rgba16Float Image that egui's Rgba8UnormSrgb pipeline
+/// can't render into.
+fn disable_egui_auto_context(mut settings: ResMut<bevy_egui::EguiGlobalSettings>) {
+    settings.auto_create_primary_context = false;
+}
+
 pub struct BlackHolePlugin;
 
 impl Plugin for BlackHolePlugin {
@@ -92,6 +101,14 @@ impl Plugin for BlackHolePlugin {
             .add_plugins(Material2dPlugin::<crate::render::material::BrightPassMaterial>::default())
             .add_plugins(Material2dPlugin::<crate::render::material::BlurMaterial>::default())
             .add_plugins(bevy_egui::EguiPlugin::default())
+            // The bloom pipeline spawns 7 Camera2d entities (offscreen + brightpass
+            // + 4 blur + composite). bevy_egui's auto-context assigns PrimaryEguiContext
+            // to the FIRST Added<Camera> — the offscreen camera, which renders to an
+            // Rgba16Float Image. egui's pipeline expects the window's Rgba8UnormSrgb
+            // surface → format mismatch crash. Disable auto-assignment and explicitly
+            // tag the composite (window) camera with PrimaryEguiContext (see spawn).
+            // Runs in PreStartup, before setup_primary_egui_context_system.
+            .add_systems(bevy::prelude::PreStartup, disable_egui_auto_context)
             .add_systems(Startup, spawn_fullscreen_quad)
             .add_systems(Startup, crate::scene::planets::spawn_default_planet)
             .add_systems(
@@ -238,7 +255,10 @@ fn spawn_fullscreen_quad(
         CompositeQuad,
         RenderLayers::layer(1),
     ));
-    commands.spawn((Camera2d, Msaa::Off, UpscaleCamera, Nudgable, RenderLayers::layer(1)));
+    // The composite camera renders to the window (Rgba8UnormSrgb surface).
+    // PrimaryEguiContext tells bevy_egui to render the UI onto THIS camera,
+    // not the offscreen/bloom cameras (which render to Rgba16Float Images).
+    commands.spawn((Camera2d, Msaa::Off, UpscaleCamera, Nudgable, bevy_egui::PrimaryEguiContext, RenderLayers::layer(1)));
 }
 
 /// Spawns the bloom pipeline (brightpass + blur pyramid + bloom_final).
@@ -272,7 +292,9 @@ fn spawn_bloom_pipeline(
     commands.spawn((
         Mesh2d(meshes.add(Rectangle::new(2.0, 2.0))),
         MeshMaterial2d(bp_materials.add(crate::render::material::BrightPassMaterial {
-            threshold: 1.0,
+            uniform: crate::render::material::BrightPassUniform {
+                threshold: 1.0, _pad0: 0.0, _pad1: 0.0, _pad2: 0.0,
+            },
             source: offscreen.clone(),
         })),
         Transform::default().with_scale(Vec3::new(bw as f32 / 2.0, bh as f32 / 2.0, 1.0)),
@@ -601,7 +623,7 @@ fn mirror_params(
     }
     // Update brightpass threshold (live-tunable).
     for (_, mat) in brightpass_materials.iter_mut() {
-        mat.threshold = params.bloom_threshold;
+        mat.uniform.threshold = params.bloom_threshold;
     }
     // Update composite material uniforms (bloom strength + exposure live-tunable).
     for (_, mat) in composite_materials.iter_mut() {