feat(effects): add Scratch post-process effect with reusable scratch_lines snippet

- src/shaders/render/scratch_lines.wgsl: reusable WGSL snippet registered as
  "render/scratch_lines"; call scratch_lines(uv, resolution, time)->f32 from
  any effect. Uses hash_1f from math/noise; 8 lines/frame, ~24fps flicker.
- src/effects/scratch.{wgsl,h,cc}: thin Scratch effect wrapping the snippet.
- Applied to "intro" and "rotating_cube" sequences as the final step.
- 35/35 tests passing.

handoff(Gemini): Scratch effect added. render/scratch_lines snippet is reusable.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 44 insertions, 0 deletions

diff --git a/src/effects/scratch_effect.cc b/src/effects/scratch_effect.cc
new file mode 100644
index 0000000..f49e601
--- /dev/null
+++ b/src/effects/scratch_effect.cc
@@ -0,0 +1,44 @@
+// 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);
+}