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, 1 insertions, 1 deletions

diff --git a/src/tests/gpu/test_demo_effects.cc b/src/tests/gpu/test_demo_effects.cc
index 9281413..619b9c9 100644
--- a/src/tests/gpu/test_demo_effects.cc
+++ b/src/tests/gpu/test_demo_effects.cc
@@ -89,7 +89,7 @@ static void test_post_process_effects() {
       {"ThemeModulationEffect",
        std::make_shared<ThemeModulationEffect>(fixture.ctx())},
       {"VignetteEffect", std::make_shared<VignetteEffect>(fixture.ctx())},
-      {"CNNEffect", std::make_shared<CNNEffect>(fixture.ctx(), 1)},
+      {"CNNEffect", std::make_shared<CNNEffect>(fixture.ctx())},
   };
 
   int passed = 0;