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/BuildingSystem.h

@@ -40,7 +40,6 @@ class BuildingSystem
 public:
     BuildingSystem(const GameConfig& config,
                    BeltSystem& belts,
-                   std::function<BuildingId()> allocateBuildingId,
                    std::function<void(const std::string&, QVector2D,
                                       const std::optional<ShipLayoutConfig>&)> spawnShip,
                    std::function<bool(const std::string&)> isItemUnlocked,
@@ -173,12 +172,12 @@ private:
     std::vector<Item>   rollOutputGroup(const RecipeDef& recipe);
 
     // No world data among these: the factory arrives per call (FactoryState.h). What is
-    // left are the immutable config, the transport layer, the shared RNG, and four
-    // callbacks into what this system cannot reach on its own -- the id counter and the
-    // block stock, which live on Simulation, and the entity and unlock sides.
+    // left are the immutable config, the transport layer, the shared RNG, and two
+    // callbacks into what this system genuinely cannot reach -- the entity model a
+    // finished ship is spawned into, and the unlock state an output group is tested
+    // against. Neither is factory data, so neither belongs in the state.
     const GameConfig&                              m_config;
     BeltSystem&                                    m_belts;
-    std::function<BuildingId()>                    m_allocateBuildingId;
     std::function<void(const std::string&, QVector2D,
                        const std::optional<ShipLayoutConfig>&)> m_spawnShip;
     std::function<bool(const std::string&)>        m_isItemUnlocked;