Adds Gun Turret MK2–MK5 as static upgrades (+2 range & +2 shots/sec each tier). Adds a mod option to add +4 range to all variants. No runtime scripts. Roman numeral overlays.
The normal turret group for fast-replace is ammo-turret, but you've named your fast-replace as gun-turret, making them not upgradeable with the upgrade planner, and they require you to delete the default turret to place down the mk. 2+
ent.fast_replaceable_group = "ammo-turret"
instead of ent.fast_replaceable_group = "gun-turret"
