Add concealment meter and skinned collider baker; reorganize assets

- ConcealmentMeter: standalone component measuring the percentage of the
  character volume embedded inside environment geometry (bake -> voxelize via
  column ray-stabbing parity -> classify interior voxels against box/sphere/
  capsule/convex colliders and terrain). Always-on, throttled, gizmo debug.
- SkinnedColliderBaker: sole per-frame collider bake owner for skinned rigs.
  MeshPainter now delegates baking and only borrows BakedMesh for the GPU
  raster and cache refresh, so the collider stays fresh outside paint mode.
- Move pose assets to Assets/Poses, retire unused Insect scripts to
  Assets/Unused, restore Paintman.controller; mode scripts and scene updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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