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/test/CombatSystemTest.cpp

@@ -69,7 +69,6 @@ struct CombatFixture
         , ships(cfg, admin)
         , buildings(cfg, belts,
                     [this]() { return nextBuildingId++; },
-                    [](int){},
                     [](const std::string&, QVector2D, const std::optional<ShipLayoutConfig>&) {},
                     [](const std::string&) -> bool { return true; },
                     rng)