make construction its own system (extracted from BuildingSystem)

src/lib/sim/BuildingSystem.h

@@ -104,7 +104,6 @@ public:
                                 const std::vector<ItemType>& filterB);
 
     // -- Tick hooks (called from Simulation::tick in the documented order) ---
-    void tickConstruction(FactoryState& state, Tick currentTick);
     // Advances the deconstruction queue (REQ-BLD-DECON-QUEUE): one building at a
     // time, in parallel with tickConstruction. Removes the front building and
     // credits its refund when its timer elapses.