refactor(cnn_v3): simplify sphere SDF in shadow pass, remove per-frame alloc

gbuf_shadow.wgsl — dfWithID():
- Sphere: replace inv_model local-space transform with direct world-space
  formula (length(p - center) - radius). Exact, no matrix multiply, no
  floating-point error from matrix inversion that can corrupt soft-shadow
  penumbra over 64 march steps.
- lp/scale now computed only inside the cases that need them (box/torus/plane)
  instead of eagerly for every object.

gbuffer_effect.cc — upload_scene_data():
- Replace per-frame std::vector<GBufObjectData> heap allocation with a
  file-static staging buffer s_obj_staging[256]: zero alloc per frame.

handoff(Gemini): sphere SDF now exact; shadow march should be cleaner.

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