Improvements

• Added "Idle Fleet Types" rule to Fleet Commonality.
• A fleet type is considered idle (and exempt from commonality costs) if both of the following conditions are met: a) No aircraft of the fleet type are assigned to any active routes AND b) The oldest aircraft in the fleet type was delivered no more than 45 game days ago.
• This grace period ensures flexibility for airlines adding or returning fleet types (e.g. from leases from other players) without incurring immediate costs.
• This change is now "live" in all games apart from "Airline Generations" and "History and the Future". The change will apply to these two games next week - please check your idle fleets before that.