New docs/PHASE2B_ADMIN_AND_PLATFORM_DATA.md covers the Platform API
split, Postgres schema and migration guarantees/rollback, the session
cookie architecture (including the whoami-routing-bug fix), the admin
portal, the API/security boundary, known limitations, and Phase 3+
future work.
Updates ENGINEERING_SOP.md §8, LAB_ARCHITECTURE.md, README.md, and
platform/README.md to reflect Platform API as the sole Provisioner
caller and Postgres as authoritative for identity/progress as of
Phase 2B.
