Courier bat is a bat that delivers mail at night.
It's a separate entity type that has its properties and logic altered to support deliveries.
Spawning
If a regular bat is nearby a mailbox and all requirements for delivery is met,
it will be converted into a courier bat that will navigate to the mailbox and start a delivery, similar to pigeons.
After delivery, it will be converted back into a regular bat.
Alternatively, if no regular bats are nearby, a courier bat will spawn nearby.
Spawned bats are "service" couriers - they will not persist after delivery.
Mail service also uses bats for service deliveries at night.
