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, 11 insertions, 8 deletions

diff --git a/src/gpu/effects/solarize_effect.cc b/src/gpu/effects/solarize_effect.cc
index 31b6c2e..d74d708 100644
--- a/src/gpu/effects/solarize_effect.cc
+++ b/src/gpu/effects/solarize_effect.cc
@@ -6,20 +6,23 @@
 
 // --- SolarizeEffect ---
 SolarizeEffect::SolarizeEffect(const GpuContext& ctx) : PostProcessEffect(ctx) {
-  uniforms_ =
-      gpu_create_buffer(ctx_.device, sizeof(float) * 6,
-                        WGPUBufferUsage_Uniform | WGPUBufferUsage_CopyDst);
+  uniforms_.init(ctx.device);
   pipeline_ = create_post_process_pipeline(ctx_.device, ctx_.format,
                                            solarize_shader_wgsl);
 }
 void SolarizeEffect::render(WGPURenderPassEncoder pass, float t, float b,
                             float i, float a) {
-  struct {
-    float t, b, i, a, w, h;
-  } u = {t, b, i, a, (float)width_, (float)height_};
-  wgpuQueueWriteBuffer(ctx_.queue, uniforms_.buffer, 0, &u, sizeof(u));
+  const CommonPostProcessUniforms u = {
+      .resolution = {(float)width_, (float)height_},
+      .aspect_ratio = a,
+      .time = t,
+      .beat = b,
+      .audio_intensity = i,
+  };
+  uniforms_.update(ctx_.queue, u);
   PostProcessEffect::render(pass, t, b, i, a);
 }
 void SolarizeEffect::update_bind_group(WGPUTextureView v) {
-  pp_update_bind_group(ctx_.device, pipeline_, &bind_group_, v, uniforms_);
+  pp_update_bind_group(ctx_.device, pipeline_, &bind_group_, v,
+                       uniforms_.get(), {});
 }