feat: per-pixel camera ray generation

assets/shaders/black_hole.wgsl

@@ -1,4 +1,5 @@
 #import bevy_sprite::mesh2d_vertex_output::VertexOutput
+#import "shaders/ray_gen.wgsl"
 
 struct BlackHoleUniforms {
     eye: vec4<f32>,
@@ -30,7 +31,11 @@ struct BlackHoleUniforms {
 
 @fragment
 fn fragment(in: VertexOutput) -> @location(0) vec4<f32> {
-    // Prove the camera basis + uniforms flow: tint by eye direction.
-    let n = normalize(uniforms.eye.xyz);
-    return vec4<f32>(abs(n) * 0.5 + 0.3, 1.0);
+    // in.uv is [0,1] across the quad. Center and flip y, apply aspect.
+    let aspect = uniforms.resolution.x / uniforms.resolution.y;
+    var uv = (in.uv * 2.0 - 1.0);
+    uv.x *= aspect;
+    let dir = ray_direction(uv);
+    // Visualize ray direction as a color (sanity check).
+    return vec4<f32>(abs(dir), 1.0);
 }

assets/shaders/ray_gen.wgsl

@@ -0,0 +1,14 @@
+// Builds a world-space camera ray direction for the current pixel.
+// `uv` is the pixel coordinate normalized to [-1,1] with aspect correction.
+fn ray_direction(uv: vec2<f32>) -> vec3<f32> {
+    // NOTE: `fov` is packed into the `.w` of `up` in BlackHoleUniforms
+    // (the Rust struct lays out `up: Vec3` + `fov: f32` as one vec4 block).
+    // The WGSL struct must stay exactly as-is per the task spec, so read fov
+    // from `uniforms.up.w` rather than a separate `uniforms.fov` field.
+    let tan_half_fov = tan(uniforms.up.w * 0.5);
+    let dir =
+        normalize(uniforms.forward.xyz)
+        + uniforms.right.xyz * (uv.x * tan_half_fov)
+        + uniforms.up.xyz    * (uv.y * tan_half_fov);
+    return normalize(dir);
+}