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, 8 insertions, 24 deletions

diff --git a/src/effects/gaussian_blur_effect.h b/src/effects/gaussian_blur_effect.h
index f4b8fcf..0c528c5 100644
--- a/src/effects/gaussian_blur_effect.h
+++ b/src/effects/gaussian_blur_effect.h
@@ -1,31 +1,15 @@
 // Gaussian blur effect v2 - single-pass blur
+// effect_params.p: x=strength(8.0), y=strength_audio(0.5), z=stretch(1.0)
 
 #pragma once
+#include "effects/shaders.h"
+#include "gpu/wgsl_effect.h"
 
-#include "gpu/effect.h"
-#include "gpu/uniform_helper.h"
-
-struct GaussianBlurParams {
-  float strength = 8.0f;
-  float strength_audio = 0.5f;
-  float stretch = 1.0f;
-  float _pad = 0.0f;
-};
-static_assert(sizeof(GaussianBlurParams) == 16,
-              "GaussianBlurParams must be 16 bytes");
-
-class GaussianBlur : public Effect {
- public:
+struct GaussianBlur : public WgslEffect {
   GaussianBlur(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:
-  WGPURenderPipeline pipeline_;
-  WGPUBindGroup bind_group_;
-  GaussianBlurParams blur_params_;
-  UniformBuffer<GaussianBlurParams> params_buffer_;
+               float end_time)
+      : WgslEffect(ctx, inputs, outputs, start_time, end_time,
+                   gaussian_blur_shader_wgsl, WGPULoadOp_Clear,
+                   WgslEffectParams{{8.0f, 0.5f, 1.0f, 0.0f}, {}}) {}
 };