MNS - Moog's Nether Structures

Adds the Mega Fortress and Large Arena, and requires Moog's Structure Lib 3.0.0 or newer.

Added

• Mega Fortress - a large new nether structure assembled from a jigsaw layout: roofed and roofless corridors, crossings from small up to mega, forks, corners, staircases, a raised upper level, dedicated spawner rooms, and matching dead-end caps.
• Large Arena - a new combat-focused nether structure with dedicated loot.
• Fortress garrison - themed inhabitants including fortress guards, archers, black guards, blaze sentinels, champions, and a warden of the keep.

Changed

• Now requires Moog's Structure Lib 3.0.0 or newer.
• Structure density - merged 5 groups of variant structures into shared weighted sets, reducing overall nether structure density.