move the asteroid width bound into FactoryState

src/lib/sim/FactoryState.h

@@ -45,4 +45,10 @@ struct FactoryState
     // The authority on which building owns which tile; every placement and removal
     // path claims and releases its body cells here.
     BuildingGrid                    grid;
+
+    // Current buildable asteroid width, the left bound for placement. Grows as the
+    // player buys expansions (REQ-EXP-UNLOCK). Deliberately not checksummed: it is
+    // derived from config and Simulation's expansion count, which is folded already.
+    // Seeded from config by BuildingSystem's constructor.
+    int                             asteroidWidth_tiles = 0;
 };