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, 6 deletions

diff --git a/doc/UNIFORM_BUFFER_GUIDELINES.md b/doc/UNIFORM_BUFFER_GUIDELINES.md
index c6cf9c8..45c7df3 100644
--- a/doc/UNIFORM_BUFFER_GUIDELINES.md
+++ b/doc/UNIFORM_BUFFER_GUIDELINES.md
@@ -97,15 +97,15 @@ static_assert(sizeof(CommonPostProcessUniforms) == 32,
               "CommonPostProcessUniforms must be 32 bytes for WGSL alignment");
 ```
 
-**Example (C++ GaussianBlurParams):**
+**Example (C++ WgslEffectParams — generic params for WgslEffect thin wrappers):**
 
 ```cpp
-struct GaussianBlurParams {
-  float strength = 2.0f;
-  float _pad = 0.0f;
+struct WgslEffectParams {
+  float p[4];  // vec4: generic float params (strength, scale, etc.)
+  float c[4];  // vec4: color or secondary params
 };
-static_assert(sizeof(GaussianBlurParams) == 8,
-              "GaussianBlurParams must be 8 bytes for WGSL alignment");
+static_assert(sizeof(WgslEffectParams) == 32,
+              "WgslEffectParams must be 32 bytes");
 ```
 
 **Example (C++ CameraParams):**