Introduce BaseBuilding struct. Rename Building and Unit to enum types
(Buildings, Units). Move CellTag and Players enums to the skirmish state
module and update imports and re‑exports accordingly. Add placeholder
miner module.
Introduce empty Building and Unit types and expose them
through new modules. Extend CellWidget to store optional
Unit and Building, update its constructor, and adjust board
rendering to create cells with these new fields (currently always None).