MNS - Moog's Nether Structures

Adds the Mega Fortress 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. - \*\*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.