add BuildingGrid to manage tile occupancy

src/lib/sim/BuildingSystem.h

@@ -15,6 +15,7 @@
 
 #include "BeltSystem.h"
 #include "Building.h"
+#include "BuildingGrid.h"
 #include "BuildingType.h"
 #include "BuildingId.h"
 #include "GameConfig.h"
@@ -309,6 +310,7 @@ private:
     };
     std::deque<DeconstructionEntry> m_deconstructionQueue;
 
-    // Maps every occupied body-cell coordinate to the entity that owns it.
-    std::map<std::pair<int, int>, BuildingId> m_tileOccupancy;
+    // The authority on which building owns which tile; every placement and removal
+    // path claims and releases its body cells here.
+    BuildingGrid m_grid;
 };