fix: remove unit from plan-driven IDs on removal

Ensure that when a unit is removed from the game, its ID is also deleted from the plan-driven unit IDs set

src/core/game/GameImpl.ts

@@ -893,6 +893,7 @@ export class GameImpl implements Game {
   }
   removeUnit(u: Unit) {
     this.unitGrid.removeUnit(u);
+    this.planDrivenUnitIds.delete(u.id());
     if (u.hasTrainStation()) {
       this._railNetwork.removeStation(u);
     }