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, 11 deletions

diff --git a/src/effects/gaussian_blur.wgsl b/src/effects/gaussian_blur.wgsl
index 68e0720..de0f2cf 100644
--- a/src/effects/gaussian_blur.wgsl
+++ b/src/effects/gaussian_blur.wgsl
@@ -6,21 +6,16 @@
 #include "common_uniforms"
 #include "render/fullscreen_vs"
 
-struct GaussianBlurParams {
-    strength: f32,
-    strength_audio: f32,
-    stretch: f32,  // dir_y / dir_x
-    _pad: f32,
-};
+// effect_params.p: x=strength, y=strength_audio, z=stretch
+struct WgslEffectParams { p: vec4f, c: vec4f }
 
 @group(0) @binding(2) var<uniform> uniforms: CommonUniforms;
-@group(0) @binding(3) var<uniform> params: GaussianBlurParams;
+@group(0) @binding(3) var<uniform> effect_params: WgslEffectParams;
 
 @fragment fn fs_main(@builtin(position) p: vec4f) -> @location(0) vec4f {
-    // Parameterized strength + dramatic beat pulsation
-    let pulse = 1.0 + uniforms.audio_intensity * params.strength_audio;  // Pulsate beat
-    let size = params.strength * pulse;
-    let dir = vec2f(1., params.stretch) * size / uniforms.resolution.x;
+    let pulse = 1.0 + uniforms.audio_intensity * effect_params.p.y;
+    let size = effect_params.p.x * pulse;
+    let dir = vec2f(1., effect_params.p.z) * size / uniforms.resolution.x;
 
     let uv = p.xy / uniforms.resolution;
     var res = vec4f(0.0);