feat(gbuffer): wire_dag() + find_downstream_output() for temporal feedback

- Add Effect::wire_dag() virtual (called from init_effect_nodes after full DAG built)
- Add Effect::find_downstream_output() protected helper (first downstream consumer output)
- GBufferEffect::wire_dag() auto-sets cnn_output_node_ via find_downstream_output,
  guarding against sink (external view, null texture)
- GBufferEffect::post_render() null-checks src texture before CopyTextureToTexture
- Tests: find_downstream_output cases + wire_dag integration in test_effect_base
- Doc: SEQUENCE.md updated with wire_dag pattern, helper contract, and sink guard

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

2 files changed, 13 insertions, 1 deletions

diff --git a/cnn_v3/src/gbuffer_effect.cc b/cnn_v3/src/gbuffer_effect.cc
index 512843c..1bfd685 100644
--- a/cnn_v3/src/gbuffer_effect.cc
+++ b/cnn_v3/src/gbuffer_effect.cc
@@ -773,10 +773,18 @@ void GBufferEffect::update_raster_bind_group(NodeRegistry& nodes) {
   wgpuBindGroupLayoutRelease(bgl);
 }
 
+void GBufferEffect::wire_dag(const std::vector<EffectDAGNode>& dag) {
+  const std::string out = find_downstream_output(dag);
+  // "sink" is an external view (no owned texture) — not a valid copy source.
+  if (out != "sink") cnn_output_node_ = out;
+}
+
 void GBufferEffect::post_render(WGPUCommandEncoder encoder, NodeRegistry& nodes) {
   if (cnn_output_node_.empty() || !nodes.has_node(cnn_output_node_)) return;
+  WGPUTexture src_tex = nodes.get_texture(cnn_output_node_);
+  if (!src_tex) return;  // external view (e.g. sink) — no owned texture to copy
   WGPUTexelCopyTextureInfo src = {};
-  src.texture  = nodes.get_texture(cnn_output_node_);
+  src.texture  = src_tex;
   src.mipLevel = 0;
   WGPUTexelCopyTextureInfo dst = {};
   dst.texture  = nodes.get_texture(node_prev_tex_);
diff --git a/cnn_v3/src/gbuffer_effect.h b/cnn_v3/src/gbuffer_effect.h
index 8e777f8..76d4347 100644
--- a/cnn_v3/src/gbuffer_effect.h
+++ b/cnn_v3/src/gbuffer_effect.h
@@ -49,6 +49,10 @@ class GBufferEffect : public Effect {
   // Copies cnn_output_node_ → node_prev_tex_ after all effects have rendered.
   void post_render(WGPUCommandEncoder encoder, NodeRegistry& nodes) override;
 
+  // Auto-wires cnn_output_node_: finds the first downstream effect whose
+  // input_nodes intersect our output_nodes, and uses its output_nodes[0].
+  void wire_dag(const std::vector<EffectDAGNode>& dag) override;
+
   // Populate the internal scene with ~20 rotating cubes and a few pumping
   // spheres.  Must be called once before the first render().
   void set_scene();