separate what buildings are from what flows through them

BuildingSystem was four unrelated jobs in one class: building lifecycle,
building configuration, the per-tick material flow, and (until last commit) the
checksum. The flow was the odd one out -- it is what the RNG, the ship spawner
and the unlock test were held for, none of which placement, rotation or
demolition has any business reaching.

Tick steps 3 to 5 move to a new ProductionSystem: tickBeltPull, tickProduction,
tickShipyardProduction, tickOutputBelts, their five private helpers and
rollOutputGroup. The cut is clean in both directions -- nothing in the block
called a topology or configuration method, and nothing outside it called the
helpers -- so the bodies move verbatim; a scripted comparison against the old
file confirms all nine differ only by class qualifier, the m_belts -> belts
rename, and the two added parameters. (Seven em dashes in comments became `--`;
the new file is ASCII, as the guidelines require.)

Belts arrive per tick rather than being held, matching ConstructionSystem, and
only the two methods that touch them take the parameter. BuildingSystem is left
holding the config and the belts, and its constructor takes exactly those two.
The arena constructs no ProductionSystem at all: it stages ships directly and
never runs a factory.

Determinism rests on the RNG stream: step 4's weighted output-group pick is the
factory's only draw, so the four calls must keep their order and position in
Simulation::tick. They do, and the tick-order section now says why, since no
test can catch a reordering here.

913 lines of BuildingSystem.cpp become 444 there and 483 in ProductionSystem.cpp.

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

src/lib/sim/Simulation.h

@@ -27,6 +27,7 @@ class AiSystem;
 class BuildingSystem;
 class ConstructionSystem;
 class DeconstructionSystem;
+class ProductionSystem;
 struct Command;
 class Hasher;
 class CombatSystem;
@@ -218,6 +219,7 @@ private:
     FactoryState                      m_factoryState;
     BeltSystem                        m_beltSystem;
     std::unique_ptr<BuildingSystem>   m_buildingSystem;
+    std::unique_ptr<ProductionSystem>   m_productionSystem;
     std::unique_ptr<ConstructionSystem> m_constructionSystem;
     std::unique_ptr<DeconstructionSystem> m_deconstructionSystem;
     std::unique_ptr<ShipSystem>       m_shipSystem;