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, 9 insertions, 0 deletions

diff --git a/doc/COMPLETED.md b/doc/COMPLETED.md
index d72591f..5d8603f 100644
--- a/doc/COMPLETED.md
+++ b/doc/COMPLETED.md
@@ -29,6 +29,15 @@ Detailed historical documents have been moved to `doc/archive/` for reference:
 
 Use `read @doc/archive/FILENAME.md` to access archived documents.
 
+## Recently Completed (March 2026)
+
+- [x] **Scratch post-process effect**
+    - New `Scratch` effect overlays random horizontal film-scratch lines additively on top of any input buffer.
+    - Scratch logic extracted into reusable snippet `src/shaders/render/scratch_lines.wgsl` (registered as `"render/scratch_lines"`). Any effect can `#include "render/scratch_lines"` and call `scratch_lines(uv, resolution, time) -> f32`. Uses `hash_1f` from `math/noise`.
+    - Applied to first two timeline sequences ("intro", "rotating_cube") as the final post-process step.
+    - **Files:** `src/shaders/render/scratch_lines.wgsl` (new), `src/effects/scratch.wgsl` (new), `src/effects/scratch_effect.{h,cc}` (new), `assets.txt`, `DemoSourceLists.cmake`, `demo_effects.h`, `shaders.{h,cc}`, `test_demo_effects.cc`, `timeline.seq`
+    - **Tests:** 35/35 passing
+
 ## Recently Completed (March 2, 2026)
 
 - [x] **OLA-IDCT Synthesis — click-free .spec decoding**