refactor: Simplify effect render API and fix uniform initialization

Root cause: Uniform buffers created but not initialized before bind group
creation, causing undefined UV coordinates in circle_mask_compute.wgsl.

Changes:
- Add get_common_uniforms() helper to Effect base class
- Refactor render()/compute() signatures: 5 params → CommonPostProcessUniforms&
- Fix uninitialized uniforms in CircleMaskEffect and CNNEffect
- Update all 19 effect implementations and headers
- Fix WGSL syntax error in FlashEffect (u.audio_intensity → audio_intensity)
- Update test files (test_sequence.cc)

Benefits:
- Cleaner API: construct uniforms once per frame, reuse across effects
- More maintainable: CommonPostProcessUniforms changes need no call site updates
- Fixes UV coordinate bug in circle_mask_compute.wgsl

All 36 tests passing (100%)

handoff(Claude): Effect API refactor complete

1 files changed, 6 insertions, 0 deletions

diff --git a/doc/EFFECT_WORKFLOW.md b/doc/EFFECT_WORKFLOW.md
index 45c47b7..d68d148 100644
--- a/doc/EFFECT_WORKFLOW.md
+++ b/doc/EFFECT_WORKFLOW.md
@@ -31,6 +31,12 @@ Automated checklist for adding new visual effects to the demo.
 - Post-process effects: inherit from `PostProcessEffect`
 - Scene effects: inherit from `Effect`
 
+**Render Signature:**
+```cpp
+void render(WGPURenderPassEncoder pass,
+            const CommonPostProcessUniforms& uniforms) override;
+```
+
 **Template:** See `tools/shadertoy/template.*` or use `convert_shadertoy.py`
 
 ### 2. Add Shader to Assets