feat: implement per-enemy damage application (spec §3.2)

- Add applyDamageToRoom() function targeting individual enemies
- Add lowestHPEnemy() helper for focus-fire targeting
- AoE spells distribute damage across all enemies
- Single-target attacks hit lowest HP enemy first
- Refactor processCombatTick spell/equipment/melee/DoT damage to use per-enemy system
- Update golemApplyDamageToRoom in golem-combat pipeline for per-enemy targeting
- Add currentRoom to CombatTickResult and sync through gameStore
- Update combat-actions tests with proper enemy setup for per-enemy tests
- Extract combat-damage.ts module to stay under 400-line limit

docs/project-structure.txt

@@ -354,6 +354,7 @@ Mana-Loop/
 │   │   │   │   │   └── pact-ritual.ts
 │   │   │   │   ├── attunementStore.ts
 │   │   │   │   ├── combat-actions.ts
+│   │   │   │   ├── combat-damage.ts
 │   │   │   │   ├── combat-descent-actions.ts
 │   │   │   │   ├── combat-state.types.ts
 │   │   │   │   ├── combatStore.ts