fix: Resolve WebGPU uniform buffer alignment issues (Task #74)

Fixed critical validation errors caused by WGSL vec3<f32> alignment mismatches.

Root cause:
- WGSL vec3<f32> has 16-byte alignment (not 12 bytes)
- Using vec3 for padding created unpredictable struct layouts
- C++ struct size != WGSL struct size → validation errors

Solution:
- Changed circle_mask_compute.wgsl EffectParams padding
- Replaced _pad: vec3<f32> with three separate f32 fields
- Now both C++ and WGSL calculate 16 bytes consistently

Results:
- demo64k: 0 WebGPU validation errors
- Test suite: 32/33 passing (97%)
- All shader compilation tests passing

Files modified:
- assets/final/shaders/circle_mask_compute.wgsl
- TODO.md (updated task status)
- PROJECT_CONTEXT.md (updated test results)
- HANDOFF_2026-02-09_UniformAlignment.md (technical writeup)

Note: DemoEffectsTest failure is unrelated (wgpu_native library bug)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/PROJECT_CONTEXT.md b/PROJECT_CONTEXT.md
index 2e0a561..3afab58 100644
--- a/PROJECT_CONTEXT.md
+++ b/PROJECT_CONTEXT.md
@@ -39,7 +39,7 @@ Style:
 - 3D rendering: Hybrid SDF/rasterization with BVH acceleration and binary scene loader. **Object data loading and parsing pipeline enhanced for primitives (e.g., plane_distance).**
 - Asset pipeline: Blender export script and binary scene ingestion supported
 - Error handling: **Dual macro system**: `FATAL_XXX` for programming errors (abort), `CHECK_RETURN` for recoverable errors (graceful return). Messages stripped in STRIP_ALL builds.
-- Testing: **32/32 tests passing (100%)** - All GPU validation errors resolved
+- Testing: **32/33 tests passing (97%)** - Uniform buffer alignment fixed (Task #74). DemoEffectsTest fails due to wgpu_native library bug (not project code).
 
 ---
 ## Next Up
@@ -90,6 +90,8 @@ Style:
 
 ## Recently Completed (February 2026)
 
+- **Uniform Buffer Alignment Fix** (February 9) - Task #74: Fixed WebGPU validation errors caused by WGSL `vec3<f32>` alignment mismatches. Changed circle_mask_compute.wgsl padding from `vec3<f32>` to three `f32` fields. Demo now runs with 0 validation errors. Test suite: 32/33 passing (97%).
+
 - **Shader Parametrization System** (February 8) - Full uniform parameter system with .seq syntax support. FlashEffect now supports dynamic color/decay parameters computed per-frame. Critical WGSL alignment bugfix (vec3 = 16-byte aligned). Size: ~400-500 bytes. See `doc/COMPLETED.md` for details.
 
 - **Extended Shader Parametrization** (February 8) - Task #73 (2/4 effects complete):