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/__tests__/combat-actions.test.ts

@@ -75,6 +75,7 @@ function runCombatTick(rawMana: number, elements: Record<string, { current: numb
     [],   // signedPacts
     { activeGolems: [] }, // golemancyState
     golemApplyDamageToRoom,
+    (dmg: number) => dmg, // applyEnemyDefenses (passthrough for tests)
   );
 }
 
@@ -94,6 +95,7 @@ function processCombatTickDirect(
     onFloorCleared, onDamageDealt, signedPacts,
     { activeGolems: [] },
     golemApplyDamageToRoom,
+    (dmg: number) => dmg, // applyEnemyDefenses (passthrough for tests)
   );
 }