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, 0 insertions, 44 deletions

diff --git a/src/effects/scratch_effect.cc b/src/effects/scratch_effect.cc
deleted file mode 100644
index f49e601..0000000
--- a/src/effects/scratch_effect.cc
+++ /dev/null
@@ -1,44 +0,0 @@
-// Scratch effect - film scratch overlay post-process
-
-#include "effects/scratch_effect.h"
-#include "effects/shaders.h"
-#include "gpu/post_process_helper.h"
-#include "util/fatal_error.h"
-
-Scratch::Scratch(const GpuContext& ctx, const std::vector<std::string>& inputs,
-                 const std::vector<std::string>& outputs, float start_time,
-                 float end_time)
-    : Effect(ctx, inputs, outputs, start_time, end_time) {
-  HEADLESS_RETURN_IF_NULL(ctx_.device);
-
-  create_linear_sampler();
-
-  pipeline_.set(create_post_process_pipeline(ctx_.device,
-                                             WGPUTextureFormat_RGBA8Unorm,
-                                             scratch_shader_wgsl));
-}
-
-void Scratch::render(WGPUCommandEncoder encoder,
-                     const UniformsSequenceParams& params,
-                     NodeRegistry& nodes) {
-  WGPUTextureView input_view = nodes.get_view(input_nodes_[0]);
-  WGPUTextureView output_view = nodes.get_view(output_nodes_[0]);
-
-  pp_update_bind_group(ctx_.device, pipeline_.get(), bind_group_.get_address(),
-                       input_view, uniforms_buffer_.get(), {nullptr, 0});
-
-  WGPURenderPassColorAttachment color_attachment = {};
-  gpu_init_color_attachment(color_attachment, output_view);
-
-  WGPURenderPassDescriptor pass_desc = {};
-  pass_desc.colorAttachmentCount = 1;
-  pass_desc.colorAttachments = &color_attachment;
-
-  WGPURenderPassEncoder pass =
-      wgpuCommandEncoderBeginRenderPass(encoder, &pass_desc);
-  wgpuRenderPassEncoderSetPipeline(pass, pipeline_.get());
-  wgpuRenderPassEncoderSetBindGroup(pass, 0, bind_group_.get(), 0, nullptr);
-  wgpuRenderPassEncoderDraw(pass, 3, 1, 0, 0);
-  wgpuRenderPassEncoderEnd(pass);
-  wgpuRenderPassEncoderRelease(pass);
-}