Something that might need looking at:
I started with a Tundra world with -1 farming.
I built the mirrors, and it upgraded the world to Terran while giving it +1 farming.
I then removed the mirrors just to see what would happen.
The planet stayed Terran while re-gaining it's previous cold condition. But lost it's farmland condition entirely.
And yet it's now producing 15 food as a size 5 colony.
Looks like the new conditions are being added whenever the mirror status changes, but the old ones are not being removed.
And the farmland is also being removed. (But still 'working'.)
Unrelated feedback/questions:
Possibly consider not changing the planet type, but improve the one that's already there. Having Terran planets potentially everywhere just doesn't "feel" right. (In this setting at least.)
Is everything in cat_barren completely untouched by these buildings?
Ie; Terraforming limited to already habitable planets.
(Fine with this btw.)
Possibly consider adding ability to frozen worlds to build Ismara's Sling.
Possibly consider adding information to the building tooltip when terraforming is stalled.
- Will this support Tart's unknown skies mod?
It would appear not. Possibly due to defing abilities by specific planet type, rather than by planet class.
Not a single US planet I tested with had the ability to build these buildings.