unreachable_protects that only protects unreachable units

[lmoureaux]

Is your feature request related to a problem? Please describe.
Currently, unreachable_protects makes a stack with a mech and a fighter is very strong: the mech defends against air attacks, and land units cannot attack.

Describe the solution you'd like
An intermediate value unreachable_protect=UNREACHABLE where only unreachable units are protected. That is, a tank could kill the mech, and fighters clear the sky afterwards.

Describe alternatives you've considered
Effects, but I don't think they would work in this context. Lua, but too complicated.

Additional context
@ec429 would like more control for Aviation.

[XHawk87]

How does it work right now with unreachableprotects=disabled? Does it mainly affect how killstack operates? I.e. If you have a mech and an engineer on a tile with a fighter above, and someone attacks it with a tank and beats the mech, if killstack is enabled all three die, and if it's off only the mech dies? If it's just a diplomat and a fighter and a tank attacks, would the fighter defend because it has better defence than a diplomat even though it's supposed to be unreachable?

[XHawk87]

To expand on this concept, it might also be useful if unreachable units can occupy the same tile even if not allied. E.g. Player A could control the ground while Player B could control the air. Or a spy could move right past enemy military units without detection. This would only work if they are both mutually-unreachable to eachother.