fix: melee attacks now apply enemy defenses (armor/barrier/dodge)

Bug: Melee sword attacks passed null as the enemy to applyEnemyDefenses,
causing all enemy defenses (armor, barrier, dodge) to be bypassed.
Spell damage and DoT effects correctly went through defenses.

Fix: In combat-actions.ts melee loop, get the current target enemy
from currentRoom.enemies (lowest HP, matching focus-fire targeting)
and pass it to applyEnemyDefenses instead of null.

Added 7 regression tests in melee-defense-bypass.test.ts to verify:
- Melee damage is reduced by armor
- Melee damage is reduced by barrier
- Unarmored enemies take more damage than armored ones
- Full damage dealt when no defenses
- Focus-fire targeting (lowest HP enemy)
- Graceful handling of empty enemy list
- Comparison proving defense application

All 948 tests pass (49 test files).

src/lib/game/stores/combat-actions.ts

@@ -270,7 +270,13 @@ export function processCombatTick(
         let meleeSafetyCounter = 0;
         while (meleeProgress >= 1 && meleeSafetyCounter < 100) {
           const meleeDamage = calcMeleeDamage(swordInstance as EquipmentInstance, swordType, floorElems[0]);
-          const finalMeleeDamage = applyEnemyDefenses(meleeDamage, null, 'combat', (msg) => logMessages.push(msg));
+          // Get the current target enemy (lowest HP, matching focus-fire targeting in applyDamageToRoom)
+          const currentRoomState = get().currentRoom;
+          const livingEnemies = currentRoomState.enemies.filter(e => e.hp > 0);
+          const targetEnemy = livingEnemies.length > 0
+            ? livingEnemies.reduce((lowest, e) => e.hp < lowest.hp ? e : lowest)
+            : null;
+          const finalMeleeDamage = applyEnemyDefenses(meleeDamage, targetEnemy, currentRoomState.roomType, (msg) => logMessages.push(msg));
           if (!Number.isFinite(finalMeleeDamage)) break;
 
           // Apply melee damage per-enemy (spec §3.2, focus-fire)