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>
Introduces AcDream.Core.World.Cells namespace with the two foundational
types for the Unified Cell Graph. CellPortal is a readonly struct
unifying the three legacy portal representations; ObjCell is the abstract
base for all traversable cells with the retail id-magnitude IsEnv
discriminator (CObjCell::GetVisible, pseudo_c:308215). Zero consumers;
zero behavior change. 5/5 tests green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
