tensura skill books

- Random skill selection and rewards are fully server-authoritative.
- Every discovered skill is generated by category at equal weight 1; set an exact skill ID to @0 to exclude it only from random rolls.
- Every explicit skill study book has its own configurable natural-loot weight.
- Discovers loaded LootTables from vanilla, mods, data packs, structures, terrain, archaeology, entities, and Lootr-related sources.
- Loot integration is enabled for discovery, but all newly generated table chances and mod-item weights default to zero. No mod item appears naturally until the server owner opts in.
- Stores configuration, random weights, loot weights, discovered tables, examples, and formula documentation under config/tensura_skill_books/.
- Uses standard server LootTables and LootPools for Lootr-compatible per-player containers.