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/lib/sim/Simulation.h

@@ -169,7 +169,6 @@ private:
 
 	void handleEvent(std::shared_ptr<const TracePrintRequestedEvent> event) override;
 
-    BuildingId allocateBuildingId();  // Strictly increasing; never returns kInvalidBuildingId.
 
     // (Re-)create every owned subsystem. Shared by the constructor and reset();
     // the construction order is load-bearing for determinism, so both paths must
@@ -195,7 +194,6 @@ private:
 
     Tick     m_currentTick;
     Tick     m_nextDepartureTick;
-    BuildingId m_nextBuildingId;
     int      m_expansionsPurchased = 0;  // REQ-EXP-COST formula variable x
     bool     m_gameOver    = false;
     bool     m_isWon       = false;