put the block stock where the blocks are

The global building block stock lived on Simulation while being factory data
through and through: placement spends it, deconstruction refunds it, and blocks
delivered to the HQ by belt add to it. Every system that credited it therefore
held a std::function back into Simulation to do so -- BuildingSystem and
DeconstructionSystem each carried one, and the arena and three test fixtures had
to pass a stub.

It moves into FactoryState, seeded by makeFactoryState from
world.starting_building_blocks, and both callbacks are gone: the HQ's belt intake
and the deconstruction refund now credit the state they are already holding.

BuildingSystem::deconstruct stops returning a refund for its caller to remember
to credit. It had grown asymmetric -- the queued path credits itself through
DeconstructionSystem while the instant path handed a number back -- so it now
credits the site's full cost directly and returns void.

Checksum order is untouched: Simulation still folds the stock at exactly the
point it always did, reading it from the state. The arena no longer discards
refunds into a no-op sink; nothing there reads the stock either way.

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

src/lib/sim/Simulation.h

@@ -196,7 +196,6 @@ private:
     Tick     m_currentTick;
     Tick     m_nextDepartureTick;
     BuildingId m_nextBuildingId;
-    int      m_buildingBlocksStock;
     int      m_expansionsPurchased = 0;  // REQ-EXP-COST formula variable x
     bool     m_gameOver    = false;
     bool     m_isWon       = false;