Remove inherited Explorer SP gates except approved level-1 dependencies

Explorer skill upgrades currently inherit class-tree gates that obstruct the approved classless progression. Retain only the owner's 17 reviewed target relationships at rank 1, with Transformation OR Super Transformation for Shockwave; remove all other Explorer SP prerequisites. Server enforcement and NPC requirement prose now use the same effective groups, including alias-aware learned/expiry checks. Combat states, resources, weapons/stats, frozen offers and claims retain their existing rules.

The exact table and removal coverage are documented in docs/explorer-prerequisites.md. Pair with the client-17 merge request for native eligibility/description agreement; the HP/MP description companion is the server-18 merge request. Related to #17; #16 offer filtering remains deferred.

Validation: full Java compilation and the existing disposable MySQL/classless/draft suite pass. Combined server source also passes 1,202 offline assertions, 353 WZ draft-pool assertions, 27 migration assertions, 22 NPC presentation assertions and 251 running-server packet assertions. Coverage includes all retained rank-1 alternatives, expired/missing bases, every removed Explorer gate, exact aliases, weapon variants, SP preservation, save/reload and claim faults. The paired native audit covers all 17 targets, aliases and expiry with original assets. All four server branches and both client branches combine without conflicts.

These are isolated server fixtures and offline native rendering checks, not live campaign or deployment evidence. Default base: 76e344abd7e7f5720dd0e634ae2a728e4e8da6cc.

GitLab review CI: pipeline 748 passed. No deployment jobs ran.