add logic to alter map for changed seen room terrain

given room malleability by player construction, the map actually changes based over time. it would be nice for the mapper to account for this fact rather than seen rooms remaining represented forever as they were when first revealed.

Start by locating the mapper logic that stores and renders previously seen rooms, then trace how room terrain changes after player construction. Define how changed terrain should update an existing map representation over time. Done means previously seen rooms no longer remain permanently represented as they were when first revealed.

Written by the indexing model from the issue text.