LoadedCell.BuildingId (init + internal setter) — set exactly once at
landblock load time by BuildingLoader; null when the cell isn't
part of any building (outdoor surface cells; dungeon cells not
enumerated in LandBlockInfo.Buildings).
GameWindow landblock-load path: builds BuildingRegistry from
LandBlockInfo.Buildings; stamps each cell's BuildingId; stores the
registry on _buildingRegistries[landblockId] (GameWindow-level dict)
for render-frame lookups. Note: LoadedLandblock is AcDream.Core.World
(a sealed record) — adding an App-type field there would violate
Code Structure Rule #2, so the registry is stored in a new
GameWindow-level dictionary instead. Cleanup wired in both
removeTerrain lambdas (OnLoad + OnResize paths).
drainedCells dict: the existing _pendingCells drain loop is extended
to also build a local CellId→LoadedCell dict; BuildingLoader.Build
uses this dict for the stamping pass so no second iteration is needed.
New BuildingLoaderTest verifies the stamping path. 5 BuildingLoader
tests total (4 from RR3 + 1 new).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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