feat(cnn_v3): GBufferEffect internal scene + GBufViewEffect debug wiring

GBufferEffect:
- set_scene() now owns Scene/Camera internally; no external pointers needed
- 20 randomly rotating cubes (xorshift32 seed, axis-angle animation)
- 4 pumping spheres (radius = base_r * (1 + audio_intensity * 0.8))
- Camera at (0,2.5,6) looking at origin; aspect updated per-frame
- GBufLightsUniforms: 2 directional lights (warm key + cool fill)
- object_type written to ObjectData.params.x (ready for SDF shadow)
- shadow/transp nodes cleared via zero-draw render passes (placeholder)
- bilinear sampler cached via create_linear_sampler() / sampler_.get()
- dead placeholder textures removed

GBufViewEffect:
- gbuf_view.wgsl: all channels now fully grayscale (removed color tint)
- seq_compiler.py: GBufViewEffect added to CLASS_TO_HEADER
- timeline.seq: cnn_v3_test uses GBufViewEffect -> sink for debug view

Docs: HOWTO.md §1 updated with set_scene() description + §1b implementation
plan for Pass 2 SDF shadow (shader spec, bind layout, C++ additions)

handoff(Gemini): GBufferEffect has internal scene, 36/36 tests green.
Next: implement Pass 2 shadow (gbuf_shadow.wgsl) per §1b plan in HOWTO.md.

1 files changed, 39 insertions, 12 deletions

diff --git a/cnn_v3/src/gbuffer_effect.h b/cnn_v3/src/gbuffer_effect.h
index 42fb0ec..d45be75 100644
--- a/cnn_v3/src/gbuffer_effect.h
+++ b/cnn_v3/src/gbuffer_effect.h
@@ -10,6 +10,7 @@
 #include "gpu/uniform_helper.h"
 #include "gpu/wgpu_resource.h"
 #include "util/mini_math.h"
+#include <vector>
 
 // Uniform for the pack compute shader
 struct GBufResUniforms {
@@ -20,6 +21,20 @@ struct GBufResUniforms {
 static_assert(sizeof(GBufResUniforms) == 16,
               "GBufResUniforms must be 16 bytes");
 
+// Single directional light: direction points *toward* the light source (world space).
+struct GBufLight {
+  vec4 direction;  // xyz = normalized direction toward light, w = unused
+  vec4 color;      // rgb = color, a = intensity
+};
+static_assert(sizeof(GBufLight) == 32, "GBufLight must be 32 bytes");
+
+struct GBufLightsUniforms {
+  GBufLight lights[2];
+  vec4      params;  // x = num_lights
+};
+static_assert(sizeof(GBufLightsUniforms) == 80,
+              "GBufLightsUniforms must be 80 bytes");
+
 class GBufferEffect : public Effect {
  public:
   GBufferEffect(const GpuContext& ctx, const std::vector<std::string>& inputs,
@@ -31,9 +46,22 @@ class GBufferEffect : public Effect {
   void render(WGPUCommandEncoder encoder, const UniformsSequenceParams& params,
               NodeRegistry& nodes) override;
 
-  void set_scene(const Scene* scene, const Camera* camera);
+  // Populate the internal scene with ~20 rotating cubes and a few pumping
+  // spheres.  Must be called once before the first render().
+  void set_scene();
 
  private:
+  // Per-cube animation state (axis-angle rotation)
+  struct CubeAnim {
+    vec3  axis;
+    float speed;  // radians/second, may be negative
+  };
+  // Per-sphere animation state (radius driven by audio_intensity)
+  struct SphereAnim {
+    int   obj_idx;     // index into scene_.objects
+    float base_radius;
+  };
+
   // Internal G-buffer node names
   std::string node_albedo_;
   std::string node_normal_mat_;
@@ -43,8 +71,13 @@ class GBufferEffect : public Effect {
   std::string node_feat0_;
   std::string node_feat1_;
 
-  const Scene*  scene_  = nullptr;
-  const Camera* camera_ = nullptr;
+  // Owned scene and camera — populated by set_scene()
+  Scene  scene_;
+  Camera camera_;
+  bool   scene_ready_ = false;
+
+  std::vector<CubeAnim>  cube_anims_;
+  std::vector<SphereAnim> sphere_anims_;
 
   // Pass 1: MRT rasterization pipeline
   RenderPipeline raster_pipeline_;
@@ -53,19 +86,13 @@ class GBufferEffect : public Effect {
   // Pass 4: Pack compute pipeline
   ComputePipeline  pack_pipeline_;
   BindGroup        pack_bind_group_;
-  UniformBuffer<GBufResUniforms> pack_res_uniform_;
-
-  // Placeholder textures for shadow/transp (white/black cleared once)
-  Texture     shadow_placeholder_tex_;
-  TextureView shadow_placeholder_view_;
-  Texture     transp_placeholder_tex_;
-  TextureView transp_placeholder_view_;
+  UniformBuffer<GBufResUniforms>    pack_res_uniform_;
+  UniformBuffer<GBufLightsUniforms> lights_uniform_;
 
   // GPU-side object data buffers (global uniforms + objects storage)
-  // These mirror the layout expected by gbuf_raster.wgsl
   GpuBuffer global_uniforms_buf_;
   GpuBuffer objects_buf_;
-  int       objects_buf_capacity_ = 0; // number of ObjectData slots allocated
+  int       objects_buf_capacity_ = 0;
 
   void create_raster_pipeline();
   void create_pack_pipeline();