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, 38 insertions, 0 deletions

diff --git a/src/gpu/wgsl_effect.h b/src/gpu/wgsl_effect.h
new file mode 100644
index 0000000..062f885
--- /dev/null
+++ b/src/gpu/wgsl_effect.h
@@ -0,0 +1,38 @@
+// WgslEffect: generic shader-only post-process effect.
+// Replaces boilerplate .h/.cc pairs for simple single-pass effects.
+
+#pragma once
+#include "gpu/effect.h"
+#include "gpu/uniform_helper.h"
+#include "gpu/wgpu_resource.h"
+
+// Generic per-effect uniform params (binding 3, 32 bytes).
+// Matches WgslEffectParams in WGSL: struct { p: vec4f, c: vec4f }
+// effect_params.p — 4 generic floats (strength, scale, etc.)
+// effect_params.c — color or secondary vec4
+struct WgslEffectParams {
+  float p[4];  // vec4: generic float params
+  float c[4];  // vec4: color / secondary params
+};
+static_assert(sizeof(WgslEffectParams) == 32, "WgslEffectParams must be 32 bytes");
+
+class WgslEffect : public Effect {
+ public:
+  // Mutate per-frame for dynamic parameter modulation.
+  WgslEffectParams effect_params;
+
+  WgslEffect(const GpuContext& ctx, const std::vector<std::string>& inputs,
+             const std::vector<std::string>& outputs, float start_time,
+             float end_time, const char* shader_code,
+             WGPULoadOp load_op = WGPULoadOp_Clear,
+             WgslEffectParams initial_params = {});
+
+  void render(WGPUCommandEncoder encoder, const UniformsSequenceParams& params,
+              NodeRegistry& nodes) override;
+
+ private:
+  RenderPipeline pipeline_;
+  BindGroup bind_group_;
+  UniformBuffer<WgslEffectParams> params_buffer_;
+  WGPULoadOp load_op_;
+};