BuildLoadedCell now reads the full portal polygon vertices from
cellStruct.Polygons[portal.PolygonId].VertexIds and stores them in
local-space on the LoadedCell. Empty arrays for unresolved polygons.
Same source as the ClipPlane block; no new dat read.
Unit test covers the data-class invariant (parallel indexing) since
the full integration is exercised only at runtime with live dat data.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
