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

src/lib/game/utils/combat-utils.ts

@@ -273,6 +273,55 @@ export function deductSpellCost(
   return { rawMana, elements: newElements };
 }
 
+// ─── Melee Damage Calculation (spec §4.3) ────────────────────────────────────
+
+/**
+ * Map from sword enchantment specialId to element type for elemental matchup.
+ */
+const SWORD_ENCHANT_ELEMENT: Record<string, string> = {
+  fireBlade: 'fire',
+  frostBlade: 'frost',
+  lightningBlade: 'lightning',
+  voidBlade: 'void',
+};
+
+/**
+ * Calculate melee damage for a sword attack (spec §4.3).
+ *
+ * Formula: baseDmg = sword.baseDamage + sword.elementalEnchantDamage
+ *          damage  = baseDmg × getElementalBonus(sword.enchantElement, enemy.element)
+ *
+ * No crit, no discipline damage bonus for melee in v1.
+ * attackSpeedMult from equipment does apply to meleeProgress accumulation.
+ */
+export function calcMeleeDamage(
+  swordInstance: EquipmentInstance,
+  swordType: { stats?: { baseDamage?: number } },
+  enemyElement: string,
+): number {
+  const baseDmg = swordType.stats?.baseDamage || 5;
+
+  // Determine enchant element from sword's enchantments
+  let enchantElement: string | null = null;
+  for (const ench of swordInstance.enchantments) {
+    const effectDef = ENCHANTMENT_EFFECTS[ench.effectId];
+    if (effectDef?.effect.type === 'special' && effectDef.effect.specialId) {
+      const elem = SWORD_ENCHANT_ELEMENT[effectDef.effect.specialId];
+      if (elem) {
+        enchantElement = elem;
+        break;
+      }
+    }
+  }
+
+  // Apply elemental bonus if sword has an elemental enchant
+  if (enchantElement) {
+    return baseDmg * getElementalBonus(enchantElement, enemyElement);
+  }
+
+  return baseDmg;
+}
+
 // ─── Equipment Spell Helpers ──────────────────────────────────────────────────
 
 // Return type for active equipment spells with source equipment