Harden world loading against missing void generator

[minoneer]

Summary

Fixes the silent failure where the skyblock world runs with the vanilla generator and regular terrain generates around islands (reported as "creates an overworld underneath my island"). Bukkit never persists plugin generators, and createWorld() silently returns already-loaded worlds unchanged — so a world loaded by the server (level-name) or a generator-less Multiverse registration stayed broken with no diagnostics.

• Detect & warn: WorldManager now verifies the attached generator after world load/creation and logs an actionable SEVERE banner (bukkit.yml snippet, recovery command) when it cannot be attached.
• Self-heal bukkit.yml: uSkyBlock registers worlds.<name>.generator: uSkyBlock in bukkit.yml when missing — the only mechanism covering every load path, including the default world. Add-only-when-missing, never overwrites admin values, fail-soft on read-only or unparseable files.
• Self-heal Multiverse: registrations missing the generator string are repaired (set + persisted) at startup; foreign generators are warned about but respected.
• Docs: new "World loading and the void generator" section in the admin setup guide, including symptoms and recovery via /usb chunk regen.

Explicitly excluded: persisting a flat-void stem via WorldType.FLAT — on 1.21.2+ that hardcodes sea level to -63 and breaks sea-level-anchored mob spawning (squid/dolphin/fish, glow squid, drowned, turtles) plus the guardian-habitat mechanic (same reason BSkyBlock reverted FLAT, BentoBox#2593).

Test Plan

• 17 new unit tests (WorldManagerTest, BukkitYmlGeneratorMappingTest, MultiverseCoreHookTest); full :uSkyBlock-Core:test suite green (176 tests)
• E2E on Paper 1.21.11: level-name=skyworld without mapping → boot 1 logs both banners and writes both bukkit.yml entries; boot 2 starts clean with the server loading the default world through the plugin generator
• Fresh-install auto-configuration from scratch (clean bukkit.yml, no worlds)

🤖 Generated with Claude Code

[minoneer]

CI note: the build_and_test failure is unrelated to this change — it is dependency-resolution rot surfaced by the first cold-cache build since March (old papermc.io/repo URL now returns 403, repo.mvdw-software.com is unreachable). 7699c8d0 fixes the Paper repo URL and scopes the flaky repos so they can only affect their own artifacts; Multiverse and everything else now resolve cleanly. The one remaining cold-cache failure is MVdWPlaceholderAPI:3.0.1-SNAPSHOT, whose only host is the dead mvdw repo — that dependency is being addressed properly in #134 (placeholder modernization) rather than re-hosted here.