feat(cnn_v3): GBufferEffect temporal feedback via post_render()

- Add Effect::post_render() virtual hook, called after all effects in
  the sequence have rendered each frame. Default is no-op.
- Sequence::render_effects() runs a second pass invoking post_render()
  on all DAG nodes after the render pass completes.
- GBufferEffect: declare internal node_prev_tex_ (U8X4_NORM) for
  persistent prev-frame CNN output. post_render() copies cnn_output_node_
  → node_prev_tex_ via CopyTextureToTexture. render() binds node_prev_tex_
  as prev_cnn (binding 6) — zero on frame 0 (matches training convention).
- Expose set_cnn_output_node(name) API; call once at setup.
- Drop brittle ping-pong / input_nodes_[0] fallback.
- Update doc/SEQUENCE.md: post_render() semantics, frame execution order,
  temporal feedback canonical pattern, node types table with G-buffer types.
- Update cnn_v3/docs/HOWTO.md: temporal feedback wiring section.

36/36 tests passing.

handoff(Gemini): prev.rgb temporal feedback now correct and generic.
Set set_cnn_output_node("sink") (or CNN output node name) once at setup.

1 files changed, 13 insertions, 0 deletions

diff --git a/cnn_v3/src/gbuffer_effect.h b/cnn_v3/src/gbuffer_effect.h
index 13d394d..8e777f8 100644
--- a/cnn_v3/src/gbuffer_effect.h
+++ b/cnn_v3/src/gbuffer_effect.h
@@ -46,6 +46,9 @@ class GBufferEffect : public Effect {
   void render(WGPUCommandEncoder encoder, const UniformsSequenceParams& params,
               NodeRegistry& nodes) override;
 
+  // Copies cnn_output_node_ → node_prev_tex_ after all effects have rendered.
+  void post_render(WGPUCommandEncoder encoder, NodeRegistry& nodes) 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();
@@ -68,6 +71,16 @@ class GBufferEffect : public Effect {
   std::string node_depth_;
   std::string node_shadow_;
   std::string node_transp_;
+  std::string node_prev_tex_;    // persistent prev-frame CNN output (rgba8unorm)
+
+  // Name of the CNN effect's output node; set by caller before first render.
+  // When non-empty, the CNN output is copied into node_prev_tex_ each frame.
+  std::string cnn_output_node_;
+
+ public:
+  void set_cnn_output_node(const std::string& name) { cnn_output_node_ = name; }
+
+ private:
 
   // Owned scene and camera — populated by set_scene()
   Scene  scene_;