feat: implement sword/melee auto-attack system (spec §3.1, §4.3)

- Add calcMeleeDamage() with elemental matchup for enchanted swords
- Add meleeSwordProgress per-instance accumulator to combat state
- Add melee branch in processCombatTick (no mana cost, no Executioner/Berserker)
- Add baseDamage/attackSpeed stats to all 5 sword types
- Wire equippedSwords through gameStore to combat tick pipeline
- 16 new regression tests, all 937 tests pass

docs/project-structure.txt

@@ -217,6 +217,7 @@ Mana-Loop/
 │   │   │   │   ├── formatting.test.ts
 │   │   │   │   ├── guardian-names.test.ts
 │   │   │   │   ├── mana-utils.test.ts
+│   │   │   │   ├── melee-auto-attack.test.ts
 │   │   │   │   ├── pact-utils.test.ts
 │   │   │   │   ├── persistence.test.ts
 │   │   │   │   ├── regression-fixes.test.ts