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/FactoryState.h

@@ -57,15 +57,26 @@ struct FactoryState
     // derived from config and Simulation's expansion count, which is folded already.
     // Seeded from config by BuildingSystem's constructor.
     int                             asteroidWidth_tiles = 0;
+
+    // The global building block stock (REQ-HQ-STARTING-BLOCKS, REQ-HQ-BELT-INPUT):
+    // what placement spends, what deconstruction refunds, and what blocks delivered to
+    // the HQ add to. Factory data, so it lives with the factory rather than being
+    // reached through a callback by every system that credits it.
+    //
+    // Folded into the checksum by Simulation, in the place it has always occupied
+    // (docs/replay_design.md).
+    int                             buildingBlocksStock = 0;
 };
 
-// A fresh factory for a new run: nothing built, and the asteroid bound seeded from
-// config. Every owner of a FactoryState creates it this way — the bound has no
-// sensible default without the config, so a default-constructed state would refuse
-// every placement on the asteroid.
+// A fresh factory for a new run: nothing built, the asteroid bound and the starting
+// block stock seeded from config. Every owner of a FactoryState creates it this way —
+// the bound has no sensible default without the config, so a default-constructed state
+// would refuse every placement on the asteroid, and the player would start with nothing
+// to build from.
 inline FactoryState makeFactoryState(const GameConfig& config)
 {
     FactoryState state;
-    state.asteroidWidth_tiles = config.world.regions.asteroidWidth_tiles;
+    state.asteroidWidth_tiles  = config.world.regions.asteroidWidth_tiles;
+    state.buildingBlocksStock  = config.world.startingBuildingBlocks;
     return state;
 }