make deconstruction its own system

DeconstructionSystem takes over the demolition queue: it runs the front entry's
timer and, when it elapses, removes the building, releases its tiles and credits
the partial refund. Simulation::tick calls it directly, in the position
tickDeconstruction held.

It needs no BeltSystem, unlike its construction counterpart: a belt, splitter or
tunnel end is unregistered the moment it is queued, not when the timer completes.
It does need the refund sink, so it takes the same addBuildingBlocks callback
BuildingSystem holds.

startFrontDeconstruction becomes a shared free function rather than moving:
BuildingSystem::deconstruct starts the timer when it queues the first entry, and
the system restarts it after each completion.

Stubbing the refund sink out in the test helper made two tests fail on the
refund not arriving — correctly. runTicks now threads the caller's stock through
instead.

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/Simulation.cpp

@@ -2,6 +2,7 @@
 
 #include "FactoryQueries.h"
 #include "ConstructionSystem.h"
+#include "DeconstructionSystem.h"
 #include "PlacementRules.h"
 
 #include <algorithm>
@@ -125,6 +126,8 @@ void Simulation::initializeSubsystems()
         [this](const std::string& itemId) -> bool { return isItemUnlocked(itemId); },
         m_rng);
     m_constructionSystem = std::make_unique<ConstructionSystem>(m_config);
+    m_deconstructionSystem = std::make_unique<DeconstructionSystem>(
+        m_config, [this](int amount) { m_buildingBlocksStock += amount; });
     m_shipSystem     = std::make_unique<ShipSystem>(m_config, m_admin);
     m_aiSystem       = std::make_unique<AiSystem>(m_config);
     m_movementIntentSystem = std::make_unique<MovementIntentSystem>();
@@ -246,7 +249,7 @@ void Simulation::tick()
 
     // Construction + production pipeline
     m_constructionSystem->tick(m_factoryState, m_beltSystem, m_currentTick);
-    m_buildingSystem->tickDeconstruction(m_factoryState, m_currentTick); // parallel to construction
+    m_deconstructionSystem->tick(m_factoryState, m_currentTick);        // parallel to construction
     m_buildingSystem->tickBeltPull(m_factoryState);                    // step 3
     m_buildingSystem->tickProduction(m_factoryState, m_currentTick);     // step 4
     m_buildingSystem->tickShipyardProduction(m_factoryState, m_currentTick); // step 4b