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

src/lib/game/__tests__/combat-actions.test.ts

@@ -24,7 +24,7 @@ function resetStores() {
     currentAction: 'climb',
     castProgress: 0,
     spireMode: false,
-    currentRoom: { roomType: 'combat', enemies: [] },
+    currentRoom: { roomType: 'combat', enemies: [{ id: 'enemy', name: 'Test Enemy', hp: getFloorMaxHP(1), maxHP: getFloorMaxHP(1), armor: 0, dodgeChance: 0, barrier: 0, element: 'fire', activeEffects: [], effectiveArmor: 0 }] },
     clearedFloors: {},
     climbDirection: 'up',
     isDescending: false,
@@ -143,7 +143,7 @@ describe('processCombatTick', () => {
     it('should advance room when HP reaches 0 (not last room)', () => {
       // Set floor HP very low so it's cleared in one cast
       // currentRoomIndex=0, roomsPerFloor=5 → clears room 0, advances to room 1
-      useCombatStore.setState({ floorHP: 1, castProgress: 0.99, climbDirection: 'up', currentRoomIndex: 0, roomsPerFloor: 5 });
+      useCombatStore.setState({ castProgress: 0.99, climbDirection: 'up', currentRoomIndex: 0, roomsPerFloor: 5, currentRoom: { roomType: 'combat', enemies: [{ id: 'enemy', name: 'Test Enemy', hp: 1, maxHP: 100, armor: 0, dodgeChance: 0, barrier: 0, element: 'fire', activeEffects: [], effectiveArmor: 0 }] } });
       const elements = makeInitialElements(500, {});
       const result = runCombatTick(1000, elements);
       // Room advanced, floor stays the same
@@ -154,7 +154,7 @@ describe('processCombatTick', () => {
 
     it('should advance floor when last room on floor is cleared', () => {
       // Set currentRoomIndex to last room so clearing it advances the floor
-      useCombatStore.setState({ floorHP: 1, castProgress: 0.99, climbDirection: 'up', currentRoomIndex: 4, roomsPerFloor: 5 });
+      useCombatStore.setState({ castProgress: 0.99, climbDirection: 'up', currentRoomIndex: 4, roomsPerFloor: 5, currentRoom: { roomType: 'combat', enemies: [{ id: 'enemy', name: 'Test Enemy', hp: 1, maxHP: 100, armor: 0, dodgeChance: 0, barrier: 0, element: 'fire', activeEffects: [], effectiveArmor: 0 }] } });
       const elements = makeInitialElements(500, {});
       const result = runCombatTick(1000, elements);
       expect(result.currentFloor).toBe(2);