PhysicsDataCache gains a `CellGraph` property (UCG Stage 1). The env-cell
hook is placed at the very top of CacheCellStruct — before the idempotency
guard and the null-PhysicsBSP early-return — so BSP-less cells are included
in the graph even though they are dropped from the legacy _cellStruct map.
PhysicsEngine.AddLandblock/RemoveLandblock mirror terrain registration into
the graph via a null-guarded DataCache?.CellGraph call. Zero behavior change:
CellGraph has no readers this stage.
A using-alias (UcgEnvCell / UcgCellGraph) resolves the EnvCell name
collision between AcDream.Core.World.Cells and DatReaderWriter.DBObjs.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
