feat(cnn_v3): GBufferEffect Pass 2 — SDF shadow raymarching

Implements gbuf_shadow.wgsl: fullscreen render pass that reads depth
from Pass 1, reconstructs world-space positions, evaluates a proxy-box
SDF for each object (via inv_model), computes soft shadows for both
directional lights using shadowWithStoredDistance(), and writes shadow
factor to the RGBA8Unorm node_shadow_ target consumed by gbuf_pack.wgsl.

Bind layout: B0=GlobalUniforms, B1=ObjectsBuffer (storage-read),
B2=texture_depth_2d, B3=GBufLightsUniforms.
Sky fragments (depth=1.0) are output as 1.0 (fully lit).
Falls back to clear(1.0) if pipeline is not ready.

36/36 tests pass.

handoff(Gemini): Pass 2 done. Pass 3 (transparency) still TODO.
Phase 4 (type-aware SDF) optional after visual validation.

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