move the asteroid width bound into FactoryState

It was the last piece of mutable world data BuildingSystem still owned, and the
placement queries need it: isPlacementValid reaches it through
bodyCellsWithinWorldBounds, so those queries cannot become free functions over
FactoryState while the bound lives on the system.

Left out of the checksum deliberately. It is derived from config and Simulation's
expansion count, which is folded already, so adding it would change every
checksum without adding information.

Still seeded from config by BuildingSystem's constructor, which keeps the
initialization at exactly the point it happened before; reset() clears the state
before initializeSubsystems() rebuilds the system, so the ordering holds.

Verified with a golden-checksum capture before and after — all four sample ticks
identical.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YHcUerKAZKWNvSKJxYKbnG

src/lib/sim/BuildingSystem.cpp

@@ -40,8 +40,8 @@ BuildingSystem::BuildingSystem(const GameConfig& config,
     , m_spawnShip(std::move(spawnShip))
     , m_isItemUnlocked(std::move(isItemUnlocked))
     , m_rng(rng)
-    , m_asteroidWidth_tiles(config.world.regions.asteroidWidth_tiles)
 {
+    m_state.asteroidWidth_tiles = config.world.regions.asteroidWidth_tiles;
 }
 
 // ---------------------------------------------------------------------------
@@ -340,7 +340,7 @@ bool BuildingSystem::bodyCellsWithinWorldBounds(const std::vector<QPoint>& bodyC
                                                 QPoint anchor) const
 {
     const int heightTiles = m_config.world.heightTiles;
-    const int leftEdgeX   = -m_asteroidWidth_tiles;
+    const int leftEdgeX   = -m_state.asteroidWidth_tiles;
     for (const QPoint& cell : bodyCells)
     {
         const QPoint worldCell = anchor + cell;

src/lib/sim/BuildingSystem.h

@@ -77,7 +77,7 @@ public:
     // Sets the current buildable asteroid width in tiles. Grows the left
     // placement bound as the player unlocks asteroid expansions (REQ-EXP-UNLOCK).
     // Defaults to world.regions.asteroid_width_tiles at construction.
-    void setAsteroidWidth_tiles(int widthTiles) { m_asteroidWidth_tiles = widthTiles; }
+    void setAsteroidWidth_tiles(int widthTiles) { m_state.asteroidWidth_tiles = widthTiles; }
 
     // Mark a building or construction site for demolition (REQ-BLD-DECONSTRUCT).
     // A construction site is removed instantly and the full cost is returned.
@@ -298,5 +298,4 @@ private:
                        const std::optional<ShipLayoutConfig>&)> m_spawnShip;
     std::function<bool(const std::string&)>        m_isItemUnlocked;
     std::mt19937&                                  m_rng;
-    int                                            m_asteroidWidth_tiles;
 };

src/lib/sim/FactoryState.h

@@ -45,4 +45,10 @@ struct FactoryState
     // The authority on which building owns which tile; every placement and removal
     // path claims and releases its body cells here.
     BuildingGrid                    grid;
+
+    // Current buildable asteroid width, the left bound for placement. Grows as the
+    // player buys expansions (REQ-EXP-UNLOCK). Deliberately not checksummed: it is
+    // derived from config and Simulation's expansion count, which is folded already.
+    // Seeded from config by BuildingSystem's constructor.
+    int                             asteroidWidth_tiles = 0;
 };