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, 54 insertions, 0 deletions

diff --git a/src/gpu/wgsl_effect.cc b/src/gpu/wgsl_effect.cc
new file mode 100644
index 0000000..0ce4730
--- /dev/null
+++ b/src/gpu/wgsl_effect.cc
@@ -0,0 +1,54 @@
+// WgslEffect: generic shader-only post-process effect.
+
+#include "gpu/wgsl_effect.h"
+#include "gpu/gpu.h"
+#include "gpu/post_process_helper.h"
+#include "util/fatal_error.h"
+
+WgslEffect::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,
+                       WgslEffectParams initial_params)
+    : Effect(ctx, inputs, outputs, start_time, end_time),
+      effect_params(initial_params),
+      load_op_(load_op) {
+  HEADLESS_RETURN_IF_NULL(ctx_.device);
+
+  create_linear_sampler();
+  params_buffer_.init(ctx_.device);
+
+  pipeline_.set(create_post_process_pipeline(ctx_.device,
+                                             WGPUTextureFormat_RGBA8Unorm,
+                                             shader_code));
+}
+
+void WgslEffect::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]);
+
+  params_buffer_.update(ctx_.queue, effect_params);
+
+  pp_update_bind_group(ctx_.device, pipeline_.get(), bind_group_.get_address(),
+                       input_view, uniforms_buffer_.get(),
+                       params_buffer_.get());
+
+  WGPURenderPassColorAttachment color_attachment = {};
+  gpu_init_color_attachment(color_attachment, output_view);
+  color_attachment.loadOp = load_op_;
+
+  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);
+}