Reject overlapping live battle save units

scripts/verify-battle-save-normalization.mjs

@@ -59,6 +59,7 @@ try {
     ['invalid unit exp threshold', { units: patchUnit(0, { exp: 100 }) }],
     ['invalid max-level unit exp threshold', { units: patchUnit(0, { level: 99, exp: 101 }) }],
     ['invalid unit hp', { units: patchUnit(0, { hp: 99, maxHp: 30 }) }],
+    ['duplicate live unit tile', { units: patchUnit(1, { x: validState.units[0].x, y: validState.units[0].y }) }],
     ['invalid unit x', { units: patchUnit(0, { x: 12 }) }],
     ['invalid unit direction', { units: patchUnit(0, { direction: 'down' }) }],
     ['missing equipment slot', { units: patchUnit(0, { equipment: { weapon: createEquipmentSet().weapon, armor: createEquipmentSet().armor } }) }],
@@ -116,6 +117,11 @@ try {
     'Expected save missing a current battle unit to be rejected.'
   );
 
+  assert(
+    isValidBattleSaveState({ ...validState, units: patchUnit(1, { hp: 0, x: validState.units[0].x, y: validState.units[0].y }) }, options),
+    'Expected defeated units to tolerate overlapping saved coordinates.'
+  );
+
   console.log('Verified battle save normalization and corrupted battle save rejection.');
 } finally {
   await server.close();

src/game/state/battleSaveState.ts

@@ -219,6 +219,7 @@ function isAttackIntentArray(value: unknown, options: BattleSaveValidationOption
 
 function areSavedBattleUnitsValid(units: unknown[], options: BattleSaveValidationOptions) {
   const seenUnitIds = new Set<string>();
+  const seenLiveTileKeys = new Set<string>();
 
   for (const unit of units) {
     if (!isSavedBattleUnitState(unit, options) || seenUnitIds.has(unit.id)) {
@@ -226,6 +227,13 @@ function areSavedBattleUnitsValid(units: unknown[], options: BattleSaveValidatio
     }
 
     seenUnitIds.add(unit.id);
+    if (unit.hp > 0) {
+      const tileKey = `${unit.x}:${unit.y}`;
+      if (seenLiveTileKeys.has(tileKey)) {
+        return false;
+      }
+      seenLiveTileKeys.add(tileKey);
+    }
   }
 
   if (!options.validUnitIds) {