Port ACME's EnvCellManager portal visibility system:
- New CellVisibility class: BFS portal traversal from camera cell,
portal-side clip-plane test, FindCameraCell with grace period
- LoadedCell data populated during streaming (portals, clip planes,
world/inverse transforms, local AABB from CellStruct vertices)
- WorldEntity.ParentCellId tags interior entities for filtering
- InstancedMeshRenderer.Draw accepts optional visibleCellIds set —
interior entities whose parent cell isn't visible are skipped
- Conditional depth clear between terrain and static mesh when
camera is inside a cell (ACME GameScene.cs pattern)
When camera is outdoors, all interiors render (visibleCellIds=null).
When camera enters a building, only BFS-reachable cells render.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
cffc3ee343