Sprint 2 research for indoor transitions. Documents:
- ACE EnvCell.find_transit_cells: sphere-plane + BSP containment
- ACE SortCell/BuildingObj.check_building_transit: outdoor→indoor
- PortalSide semantics and portal polygon plane testing
- Gap analysis: acdream needs CellBSP, BldPortal list, VisibleCells
Key finding: full accuracy requires CellBSP (physics BSP from dat)
for sphere_intersects_cell. Current PortalPlane.IsCrossing is a valid
approximation. ACME's AABB PointInCell is an intermediate option.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
