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, 18 insertions, 7 deletions

diff --git a/workspaces/main/shaders/cnn/cnn_layer.wgsl b/workspaces/main/shaders/cnn/cnn_layer.wgsl
index b2bab26..2285ef9 100644
--- a/workspaces/main/shaders/cnn/cnn_layer.wgsl
+++ b/workspaces/main/shaders/cnn/cnn_layer.wgsl
@@ -1,5 +1,6 @@
 // CNN layer shader - uses modular convolution snippets
 // Supports multi-pass rendering with residual connections
+// DO NOT EDIT - Generated by train_cnn.py
 
 @group(0) @binding(0) var smplr: sampler;
 @group(0) @binding(1) var txt: texture_2d<f32>;
@@ -11,12 +12,13 @@
 
 struct CNNLayerParams {
     layer_index: i32,
-    use_residual: i32,
+    blend_amount: f32,
     _pad: vec2<f32>,
 };
 
 @group(0) @binding(2) var<uniform> uniforms: CommonUniforms;
 @group(0) @binding(3) var<uniform> params: CNNLayerParams;
+@group(0) @binding(4) var original_input: texture_2d<f32>;
 
 @vertex fn vs_main(@builtin(vertex_index) i: u32) -> @builtin(position) vec4<f32> {
     var pos = array<vec2<f32>, 3>(
@@ -27,6 +29,8 @@ struct CNNLayerParams {
 
 @fragment fn fs_main(@builtin(position) p: vec4<f32>) -> @location(0) vec4<f32> {
     let uv = p.xy / uniforms.resolution;
+    let input = textureSample(txt, smplr, uv);
+    let original = textureSample(original_input, smplr, uv);
     var result = vec4<f32>(0.0);
 
     // Layer 0 uses coordinate-aware convolution
@@ -35,12 +39,19 @@ struct CNNLayerParams {
                                         rgba_weights_layer0, coord_weights_layer0, bias_layer0);
         result = cnn_tanh(result);
     }
-
-    // Residual connection
-    if (params.use_residual != 0) {
-        let input = textureSample(txt, smplr, uv);
-        result = input + result * 0.3;
+    else if (params.layer_index == 1) {
+        result = cnn_conv3x3(txt, smplr, uv, uniforms.resolution,
+                                   weights_layer1, bias_layer1);
+        result = cnn_tanh(result);
+    }
+    else if (params.layer_index == 2) {
+        result = cnn_conv3x3(txt, smplr, uv, uniforms.resolution,
+                                   weights_layer2, bias_layer2);
+    }
+    else {
+        result = input;
     }
 
-    return result;
+    // Blend with ORIGINAL input from layer 0
+    return mix(original, result, params.blend_amount);
 }