feat(effects): add Scratch post-process effect with reusable scratch_lines snippet

- src/shaders/render/scratch_lines.wgsl: reusable WGSL snippet registered as
  "render/scratch_lines"; call scratch_lines(uv, resolution, time)->f32 from
  any effect. Uses hash_1f from math/noise; 8 lines/frame, ~24fps flicker.
- src/effects/scratch.{wgsl,h,cc}: thin Scratch effect wrapping the snippet.
- Applied to "intro" and "rotating_cube" sequences as the final step.
- 35/35 tests passing.

handoff(Gemini): Scratch effect added. render/scratch_lines snippet is reusable.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 35 insertions, 0 deletions

diff --git a/src/shaders/render/scratch_lines.wgsl b/src/shaders/render/scratch_lines.wgsl
new file mode 100644
index 0000000..04ed6f8
--- /dev/null
+++ b/src/shaders/render/scratch_lines.wgsl
@@ -0,0 +1,35 @@
+// Horizontal film scratch lines utility
+// Returns scratch intensity [0,1] to additively overlay on a color buffer.
+//
+// Parameters:
+//   uv         - normalized texture coordinates [0,1]
+//   resolution - screen size in pixels (used for sub-pixel thickness)
+//   time       - physical time in seconds
+//
+// Simulates aged/damaged film: ~8 random horizontal lines per frame,
+// each with independent position, thickness, brightness and on/off state.
+// Time is quantized to 24 buckets/sec so scratches flicker like film frames.
+#include "math/noise"
+
+fn scratch_lines(uv: vec2f, resolution: vec2f, time: f32) -> f32 {
+    // Quantize to ~24fps to simulate film frame rate
+    let t = floor(time * 24.0);
+
+    var intensity = 0.0;
+    for (var i = 0; i < 8; i++) {
+        let seed = f32(i) * 17.3 + t;
+        // ~25% chance this scratch is visible this frame
+        let visible = step(0.75, hash_1f(seed + 3.1));
+        // Random vertical position [0, 1]
+        let y = hash_1f(seed + 7.9);
+        // Thickness in pixels [1, 4]
+        let thickness = hash_1f(seed + 13.5) * 3.0 + 1.0;
+        // Brightness [0.5, 1.0]
+        let brightness = hash_1f(seed + 21.7) * 0.5 + 0.5;
+        // Soft linear falloff from line center
+        let dy = abs(uv.y - y) * resolution.y;
+        let line = clamp(1.0 - dy / thickness, 0.0, 1.0);
+        intensity += line * brightness * visible;
+    }
+    return clamp(intensity, 0.0, 1.0);
+}