make BuildingSystem stateless: FactoryState becomes a parameter

src/test/CombatSystemTest.cpp

@@ -52,7 +52,7 @@ static entt::entity findWeaponChild(EntityAdmin& admin, entt::entity ship)
 struct CombatFixture
 {
     GameConfig cfg;
-    FactoryState state;
+    FactoryState state = makeFactoryState(cfg);
     std::mt19937 rng;
     EntityAdmin admin;
     BuildingId nextBuildingId;
@@ -67,7 +67,7 @@ struct CombatFixture
         , nextBuildingId(1)
         , belts(cfg.world.beltSpeed_tps)
         , ships(cfg, admin)
-        , buildings(cfg, state, belts,
+        , buildings(cfg, belts,
                     [this]() { return nextBuildingId++; },
                     [](int){},
                     [](const std::string&, QVector2D, const std::optional<ShipLayoutConfig>&) {},