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, 32 insertions, 0 deletions

diff --git a/tools/seq_compiler.cc b/tools/seq_compiler.cc
index 4a671f4..fad2d88 100644
--- a/tools/seq_compiler.cc
+++ b/tools/seq_compiler.cc
@@ -995,6 +995,38 @@ int main(int argc, char* argv[]) {
                    << eff.class_name << ">(ctx, p), " << eff.start << "f, "
                    << eff.end << "f, " << eff.priority << ");\n";
           out_file << "    }\n";
+        } else if (!eff.params.empty() && eff.class_name == "CNNEffect") {
+          // Generate parameter struct initialization for CNNEffect
+          // If layers>1, expand into multiple chained effect instances
+          int num_layers = 1;
+          float blend_amount = 1.0f;
+
+          for (const auto& [key, value] : eff.params) {
+            if (key == "layers") {
+              num_layers = std::stoi(value);
+            } else if (key == "blend") {
+              blend_amount = std::stof(value);
+            }
+          }
+
+          // Generate one effect per layer
+          for (int layer = 0; layer < num_layers; ++layer) {
+            out_file << "    {\n";
+            out_file << "      CNNEffectParams p;\n";
+            out_file << "      p.layer_index = " << layer << ";\n";
+            out_file << "      p.total_layers = " << num_layers << ";\n";
+            // Only apply blend_amount on the last layer
+            if (layer == num_layers - 1) {
+              out_file << "      p.blend_amount = " << blend_amount << "f;\n";
+            } else {
+              out_file << "      p.blend_amount = 1.0f;\n";
+            }
+            out_file << "      seq->add_effect(std::make_shared<"
+                     << eff.class_name << ">(ctx, p), " << eff.start << "f, "
+                     << eff.end << "f, " << (std::stoi(eff.priority) + layer)
+                     << ");\n";
+            out_file << "    }\n";
+          }
         } else {
           // No parameters or unsupported effect - use default constructor
           out_file << "    seq->add_effect(std::make_shared<" << eff.class_name