minor change addressing PR

src/classes/persistence/Persistence.lua

@@ -13,6 +13,7 @@ do
     ---@field unitsStates table<string, UnitState>
     ---@field random_missions table<string, MissionState>
     ---@field deliveredKilos table<string, number>
+    ---@field activeStage number? required for backwards compatibility
     ---@field activeStageInStageLane table<string, number>
 
     ---@class UnitState