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.