Adds `EnvCell` (sealed, extends `ObjCell`) with a primitive constructor
and `PointInCell` that uses the cell-containment BSP when present, else
falls back to an AABB test. Retail anchor: CEnvCell (acclient.h:32072).
BSP branch delegates to `BSPQuery.PointInsideCellBsp` (BSPQuery.cs:1034);
the AABB branch is the genuinely new logic. No `FromDat` factory — that is
a separate later task. Consumed by nobody yet (Stage 1 scaffold).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
