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).

docs/project-structure.txt

@@ -228,6 +228,7 @@ Mana-Loop/
 │   │   │   │   ├── guardian-names.test.ts
 │   │   │   │   ├── mana-utils.test.ts
 │   │   │   │   ├── melee-auto-attack.test.ts
+│   │   │   │   ├── melee-defense-bypass.test.ts
 │   │   │   │   ├── pact-utils.test.ts
 │   │   │   │   ├── persistence.test.ts
 │   │   │   │   ├── regression-fixes.test.ts