feat: implement DoT/debuff runtime system (spec §6, AC-12, AC-13)

- Add ActiveEffect, EffectType types to game.ts; activeEffects + effectiveArmor on EnemyState
- Add SpellOnHitEffect + onHitEffect field to SpellDefinition
- Wire onHitEffect to fire (burn), death (curse), lightning (armor_corrode), frost (freeze), soul (bypassArmor burn)
- Add applyOnHitEffect() — applies on-hit effect on successful spell hit (spec §6.2)
- Add processDoTPhase() — ticks all active effects after weapon/golem attacks (spec §6.3)
- Add bypassArmor/bypassBarrier support in applyEnemyDefenses() (AC-13)
- Export standalone applyEnemyDefenses from combat-tick.ts for DoT pipeline
- Split DoT runtime into separate dot-runtime.ts (135 lines) to keep combat-actions.ts under 400 lines
- Update all enemy generation sites with activeEffects/effectiveArmor defaults
- Fix test helpers for new required fields

All 921 tests pass (45 test files)

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

@@ -65,6 +65,8 @@ export function generateSwarmEnemies(floor: number): EnemyState[] {
       dodgeChance: 0,
       barrier: getEnemyBarrier(floor, element),
       element,
+      activeEffects: [],
+      effectiveArmor: SWARM_CONFIG.armorBase + Math.floor(floor / 10) * SWARM_CONFIG.armorPerFloor,
     });
   }
   return enemies;