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/stores/combat-damage.ts

@@ -0,0 +1,73 @@
+// ─── Per-Enemy Damage Application (spec §3.2) ─────────────────────────────────
+// Extracted from combat-actions.ts to stay under the 400-line file limit.
+
+import type { CombatStore, CombatState } from './combat-state.types';
+import type { EnemyState } from '../types';
+
+/**
+ * Find the enemy with the lowest current HP in the room (focus-fire targeting).
+ * Returns null if no living enemies exist.
+ */
+export function lowestHPEnemy(enemies: EnemyState[]): EnemyState | null {
+  let lowest: EnemyState | null = null;
+  for (const enemy of enemies) {
+    if (enemy.hp <= 0) continue;
+    if (!lowest || enemy.hp < lowest.hp) {
+      lowest = enemy;
+    }
+  }
+  return lowest;
+}
+
+/**
+ * Apply damage to enemies in the current room.
+ *
+ * Spec §3.2:
+ * - AoE spells: each enemy takes full damage
+ * - Single-target: target the enemy with lowest HP (focus-fire)
+ * - Recalculates floorHP as sum of all enemy HP
+ * - Triggers onRoomCleared when all enemies reach 0 HP
+ *
+ * @returns { floorHP, floorMaxHP, roomCleared }
+ */
+export function applyDamageToRoom(
+  get: () => CombatStore,
+  set: (state: Partial<CombatState>) => void,
+  dmg: number,
+  isAoe: boolean,
+): { floorHP: number; floorMaxHP: number; roomCleared: boolean } {
+  const state = get();
+  const room = state.currentRoom;
+  if (!room || room.enemies.length === 0) {
+    // No enemies in room — fall back to direct floorHP subtraction
+    const newFloorHP = Math.max(0, state.floorHP - dmg);
+    set({ floorHP: newFloorHP });
+    return { floorHP: newFloorHP, floorMaxHP: state.floorMaxHP, roomCleared: newFloorHP <= 0 };
+  }
+
+  // For single-target, find the lowest HP enemy once (focus-fire)
+  const singleTarget = isAoe ? null : lowestHPEnemy(room.enemies);
+
+  const updatedEnemies = room.enemies.map((enemy) => {
+    if (enemy.hp <= 0) return enemy;
+    if (isAoe) {
+      // AoE: each enemy takes full damage
+      return { ...enemy, hp: Math.max(0, enemy.hp - dmg) };
+    }
+    // Single-target: only damage the lowest HP enemy
+    if (singleTarget && enemy.id === singleTarget.id) {
+      return { ...enemy, hp: Math.max(0, enemy.hp - dmg) };
+    }
+    return enemy;
+  });
+
+  const newFloorHP = updatedEnemies.reduce((sum, e) => sum + e.hp, 0);
+  const allDead = updatedEnemies.every((e) => e.hp <= 0);
+
+  set({
+    currentRoom: { ...room, enemies: updatedEnemies },
+    floorHP: newFloorHP,
+  });
+
+  return { floorHP: newFloorHP, floorMaxHP: state.floorMaxHP, roomCleared: allDead };
+}