number the buildings from the factory that holds them

The building-id counter was the last piece of factory data living on Simulation
behind a callback: every construction site, every building, and every tile a
station entity claims took its id from a std::function BuildingSystem held, which
the arena and three test fixtures each had to supply.

It moves into FactoryState as nextBuildingId, handed out by
allocateBuildingId(state) in FactoryQueries beside the other operations over the
state. BuildingSystem's callback is gone; so are Simulation::allocateBuildingId
and ArenaSimulation::allocateBuildingId, whose remaining callers now allocate
from the state directly.

Checksum order is untouched: Simulation folds the counter where it always did.

What is left on BuildingSystem is the config, the belts, the RNG, and two
callbacks that reach genuinely outside the factory -- spawning a finished ship
into the entity model, and testing an output group against the unlock state.
Neither is factory data, so this is where the migration stops.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ne3mejABZoLWKLh8fgpM3x

src/balancing/ArenaSimulation.h

@@ -92,7 +92,6 @@ public:
     const EntityAdmin& getAdmin() const;
 
 private:
-    BuildingId allocateBuildingId();
     void placeStructures();
     void spawnShips();
     void computeTeamMaxEhp();
@@ -105,7 +104,6 @@ private:
     std::mt19937 m_rng;
 
     Tick m_currentTick;
-    BuildingId m_nextBuildingId;
 
     EntityAdmin m_admin;
     FactoryState                      m_factoryState;