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, 6 insertions, 0 deletions

diff --git a/src/gpu/effect.h b/src/gpu/effect.h
index 8f35f3c..f008c8d 100644
--- a/src/gpu/effect.h
+++ b/src/gpu/effect.h
@@ -44,6 +44,12 @@ class Effect {
     return false;
   }
 
+  // If true, MainSequence will capture current framebuffer to "captured_frame"
+  // auxiliary texture before rendering this effect
+  virtual bool needs_framebuffer_capture() const {
+    return false;
+  }
+
  protected:
   const GpuContext& ctx_;
   UniformBuffer<CommonPostProcessUniforms> uniforms_;