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>
