feat: Add multi-layer CNN support with framebuffer capture and blend control

Implements automatic layer chaining and generic framebuffer capture API for
multi-layer neural network effects with proper original input preservation.

Key changes:
- Effect::needs_framebuffer_capture() - generic API for pre-render capture
- MainSequence: auto-capture to "captured_frame" auxiliary texture
- CNNEffect: multi-layer support via layer_index/total_layers params
- seq_compiler: expands "layers=N" to N chained effect instances
- Shader: @binding(4) original_input available to all layers
- Training: generates layer switches and original input binding
- Blend: mix(original, result, blend_amount) uses layer 0 input

Timeline syntax: CNNEffect layers=3 blend=0.7

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 19 insertions, 3 deletions

diff --git a/src/gpu/effects/cnn_effect.h b/src/gpu/effects/cnn_effect.h
index 9cc4935..bc074d2 100644
--- a/src/gpu/effects/cnn_effect.h
+++ b/src/gpu/effects/cnn_effect.h
@@ -7,23 +7,39 @@
 
 struct CNNLayerParams {
   int layer_index;
-  int use_residual;
+  float blend_amount;  // Blend: mix(input, output, blend_amount)
   float _pad[2];
 };
 static_assert(sizeof(CNNLayerParams) == 16);
 
+struct CNNEffectParams {
+  int layer_index = 0;        // Which layer to render (0-based)
+  int total_layers = 1;       // Total number of layers in the CNN
+  float blend_amount = 1.0f;  // Final blend with original input
+};
+
 class CNNEffect : public PostProcessEffect {
  public:
-  explicit CNNEffect(const GpuContext& ctx, int num_layers = 1);
+  explicit CNNEffect(const GpuContext& ctx);
+  explicit CNNEffect(const GpuContext& ctx, const CNNEffectParams& params);
 
   void init(MainSequence* demo) override;
   void render(WGPURenderPassEncoder pass, float time, float beat,
               float intensity, float aspect_ratio) override;
   void update_bind_group(WGPUTextureView input_view) override;
 
+  // Layer 0 needs framebuffer capture for original input
+  bool needs_framebuffer_capture() const override {
+    return layer_index_ == 0;
+  }
+
  private:
-  int num_layers_;
+  int layer_index_;
+  int total_layers_;
+  float blend_amount_;
   WGPUTextureView input_view_;
+  WGPUTextureView original_view_;
   UniformBuffer<CNNLayerParams> params_buffer_;
   WGPUBindGroup bind_group_;
+  MainSequence* demo_ = nullptr;
 };