drop CombatSystem's unused BuildingSystem parameter

The parameter was already commented out in the definition — combat resolution
never touched it. Removing it also removes the last reference to BuildingSystem
from CombatSystem, so the forward declaration goes too.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YHcUerKAZKWNvSKJxYKbnG

src/lib/sim/Simulation.cpp

@@ -267,8 +267,7 @@ void Simulation::tick()
     m_repairSystem->tick(m_currentTick, m_beamFiredEvents);
 
     // Step 8: combat resolution
-    m_combatSystem->tick(m_currentTick, m_admin,
-                         *m_buildingSystem, m_beamFiredEvents);
+    m_combatSystem->tick(m_currentTick, m_admin, m_beamFiredEvents);
 
     // Step 8b: deferred damage whose impact tick has arrived
     m_combatSystem->applyPendingDamage(m_currentTick, m_admin);