feat: implement darxo spawn/upgrade policy

[Darxo]

This PR implements the following two rules:

• A UnitBlock can only spawn the weakest already present Unit
• A UnitBlock will only upgrade the weakest already present Unit

Those rules are implemented by the following changes in the code:

• When the last unit of a type is upgraded, it is deleted from the block during this SpawnProcess
• When trying to upgrade, only the weakest available unit is checked for. If that can't upgrade then this block can't upgrade

Todo:

• Gate these changes behind a bool setting so that they can be tried around with in praxis

I did a handful of test generations and it looks like the changes work as I envision them.