refactor(effects): introduce WgslEffect for shader-only post-process effects

Replace boilerplate .h/.cc pairs for simple single-pass effects with a
generic WgslEffect base class that takes a shader string + optional
WgslEffectParams (binding 3). Port Flash, Passthrough, Heptagon, Scratch,
and GaussianBlur to thin header-only wrappers — no .cc files, no CMake
entries needed. Removes 5 .cc files (-243 lines).

Update EFFECT_WORKFLOW.md, CONTRIBUTING.md, and AI_RULES.md to document
the WgslEffect (Path A) vs full class (Path B) workflow. Doc cleanup:
fix stale GaussianBlurParams/PostProcessEffect references and test counts.

handoff(Claude): WgslEffect landed; 5 effects ported; docs updated.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 6 insertions, 13 deletions

diff --git a/src/effects/scratch_effect.h b/src/effects/scratch_effect.h
index fb36a96..6c0e0cb 100644
--- a/src/effects/scratch_effect.h
+++ b/src/effects/scratch_effect.h
@@ -1,20 +1,13 @@
 // Scratch effect - film scratch overlay post-process
-// Reads input buffer, additively blends scratch_lines() on top.
 
 #pragma once
-#include "gpu/effect.h"
-#include "gpu/wgpu_resource.h"
+#include "effects/shaders.h"
+#include "gpu/wgsl_effect.h"
 
-class Scratch : public Effect {
- public:
+struct Scratch : public WgslEffect {
   Scratch(const GpuContext& ctx, const std::vector<std::string>& inputs,
           const std::vector<std::string>& outputs, float start_time,
-          float end_time);
-
-  void render(WGPUCommandEncoder encoder, const UniformsSequenceParams& params,
-              NodeRegistry& nodes) override;
-
- private:
-  RenderPipeline pipeline_;
-  BindGroup bind_group_;
+          float end_time)
+      : WgslEffect(ctx, inputs, outputs, start_time, end_time,
+                   scratch_shader_wgsl) {}
 };