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, 3 insertions, 0 deletions

diff --git a/src/tests/gpu/test_demo_effects.cc b/src/tests/gpu/test_demo_effects.cc
index 5c4bffc..3aa6230 100644
--- a/src/tests/gpu/test_demo_effects.cc
+++ b/src/tests/gpu/test_demo_effects.cc
@@ -70,6 +70,9 @@ static void test_effects() {
       {"Scene2", std::make_shared<Scene2Effect>(
                      fixture.ctx(), std::vector<std::string>{"source"},
                      std::vector<std::string>{"sink"}, 0.0f, 1000.0f)},
+      {"Scratch", std::make_shared<Scratch>(
+                      fixture.ctx(), std::vector<std::string>{"source"},
+                      std::vector<std::string>{"sink"}, 0.0f, 1000.0f)},
   };
 
   int passed = 0;