make BuildingSystem stateless: FactoryState becomes a parameter

src/lib/sim/FactoryState.h

@@ -4,6 +4,7 @@
 #include <vector>
 
 #include "Building.h"
+#include "GameConfig.h"
 #include "BuildingGrid.h"
 #include "BuildingId.h"
 #include "ItemType.h"
@@ -52,3 +53,14 @@ struct FactoryState
     // Seeded from config by BuildingSystem's constructor.
     int                             asteroidWidth_tiles = 0;
 };
+
+// A fresh factory for a new run: nothing built, and the asteroid bound seeded from
+// config. Every owner of a FactoryState creates it this way — the bound has no
+// sensible default without the config, so a default-constructed state would refuse
+// every placement on the asteroid.
+inline FactoryState makeFactoryState(const GameConfig& config)
+{
+    FactoryState state;
+    state.asteroidWidth_tiles = config.world.regions.asteroidWidth_tiles;
+    return state;
+}