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.

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