Add self-contained branch_already_chosen Bruno request, HTTP deny smoke
for unknown track/tier/branch and null-body 400, folder docs for level_too_low.
GET/POST /game/players/{id}/perk-state with versioned JSON, engine-backed
branch selection, PerkStateApiTests, Bruno collection, README, and E2.M3 alignment.
Mirror validate_content.py gates in MasteryCatalogLoader, expose IMasteryCatalogRegistry, cross-check skillIds against the skill catalog, and enforce tierIndex 1..N per track.