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

Fixed multiple WGSL/C++ struct alignment mismatches causing validation errors:

Padding fixes:
- fade_effect.cc: Changed EffectParams padding from vec3<f32> to _pad0/1/2
- theme_modulation_effect.cc: Same padding fix for EffectParams
- Root cause: WGSL vec3<f32> has 16-byte alignment, creating 32-byte structs

ODR violation fix:
- demo_effects.h: Added includes for fade_effect.h, theme_modulation_effect.h
- Removed incomplete forward declarations (88 bytes) conflicting with
  complete definitions (96 bytes), causing heap buffer overflow in make_shared

Member shadowing cleanup:
- Renamed Effect::uniforms_ shadowing members to descriptive names:
  - FadeEffect: uniforms_ -> common_uniforms_
  - FlashEffect: uniforms_ -> flash_uniforms_
  - ThemeModulationEffect: uniforms_ -> common_uniforms_

Results:
- demo64k runs without crashes
- 33/33 tests passing (100%)
- Added Task #75: WGSL uniform validation tool

handoff(Claude): Uniform buffer alignment debugged and fixed

1 files changed, 6 insertions, 4 deletions

diff --git a/src/gpu/effects/theme_modulation_effect.cc b/src/gpu/effects/theme_modulation_effect.cc
index fc52b62..f9ae636 100644
--- a/src/gpu/effects/theme_modulation_effect.cc
+++ b/src/gpu/effects/theme_modulation_effect.cc
@@ -26,7 +26,9 @@ ThemeModulationEffect::ThemeModulationEffect(const GpuContext& ctx)
 
     struct EffectParams {
       theme_brightness: f32,
-      _pad: vec3<f32>,
+      _pad0: f32,
+      _pad1: f32,
+      _pad2: f32,
     };
 
     @group(0) @binding(0) var inputSampler: sampler;
@@ -59,14 +61,14 @@ ThemeModulationEffect::ThemeModulationEffect(const GpuContext& ctx)
   pipeline_ =
       create_post_process_pipeline(ctx_.device, ctx_.format, shader_code);
 
-  uniforms_.init(ctx_.device);
+  common_uniforms_.init(ctx_.device);
   params_buffer_ = gpu_create_buffer(
       ctx_.device, 16, WGPUBufferUsage_Uniform | WGPUBufferUsage_CopyDst);
 }
 
 void ThemeModulationEffect::update_bind_group(WGPUTextureView input_view) {
   pp_update_bind_group(ctx_.device, pipeline_, &bind_group_, input_view,
-                       uniforms_.get(), params_buffer_);
+                       common_uniforms_.get(), params_buffer_);
 }
 
 void ThemeModulationEffect::render(WGPURenderPassEncoder pass, float time,
@@ -79,7 +81,7 @@ void ThemeModulationEffect::render(WGPURenderPassEncoder pass, float time,
       .beat = beat,
       .audio_intensity = intensity,
   };
-  uniforms_.update(ctx_.queue, u);
+  common_uniforms_.update(ctx_.queue, u);
 
   // Alternate between bright and dark every 4 seconds (2 pattern changes)
   // Music patterns change every 2 seconds at 120 BPM