Hero ships aren't hidden once two are in fleet

Currently, Hero ships are hidden based on their Unique keyword. They should also be hidden once any two Hero ships are already in the fleet. Not currently a concern as there is no way to take more than two in a fleet.

Locate the fleet data entries that define Hero ships and their Unique keyword, then inspect how fleet availability is represented. Done means Hero ships remain hidden when any two Hero ships are already in the fleet, while the existing Unique-based hiding still works.

Written by the indexing model from the issue text.