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>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/gpu/sequence.cc b/src/gpu/sequence.cc
index 78647b2..6bff34e 100644
--- a/src/gpu/sequence.cc
+++ b/src/gpu/sequence.cc
@@ -291,4 +291,7 @@ void Sequence::init_effect_nodes() {
   for (auto& dag_node : effect_dag_) {
     dag_node.effect->declare_nodes(nodes_);
   }
+  for (auto& dag_node : effect_dag_) {
+    dag_node.effect->wire_dag(effect_dag_);
+  }
 }