Validate battle save campaign step

src/game/state/battleSaveState.ts

@@ -1,5 +1,5 @@
 import type { BattleBond, UnitData } from '../data/scenario';
-import type { CampaignStep } from './campaignState';
+import { isCampaignStep, type CampaignStep } from './campaignState';
 import type { UnitDirection } from '../data/unitAssets';
 import { equipmentExpToNext, equipmentSlots, type EquipmentSlot } from '../data/battleItems';
 
@@ -142,6 +142,10 @@ export function isValidBattleSaveState(state: unknown, options: BattleSaveValida
     return false;
   }
 
+  if (state.campaignStep !== undefined && !isCampaignStep(state.campaignStep)) {
+    return false;
+  }
+
   const attackIntents = state.attackIntents;
   const units = state.units;