move FactoryState ownership out of BuildingSystem to Simulation

src/lib/sim/FactoryState.h

@@ -31,8 +31,11 @@ struct DeconstructionEntry
 // data/behaviour split the ecs/system/ classes already follow, where world data
 // arrives as a tick argument instead of being owned by the system.
 //
-// Owned by BuildingSystem for now. The intent is to hand ownership to Simulation
-// and pass this into the tick methods, so the systems become stateless over it.
+// Owned by Simulation (and by ArenaSimulation in the balancing tool), not by the
+// systems that operate on it. BuildingSystem holds a reference. The remaining step
+// is to pass this into the tick methods instead, so the systems become stateless
+// over it — that one is gated on the query surface, which today reaches the data
+// through BuildingSystem's ~180 const call sites.
 struct FactoryState
 {
     std::vector<Building>           buildings;