docs(phase10): Backend Configuration reference + Roadmap/Home/Sessions IMPLEMENTED Phase 10 (tunable-backend config surface) landed → EHDB program (Phases 6-10) code-complete. New Backend-Configuration page (per-tier selection, env vars, precedence, coherent/incoherent examples, ehdb-selfcheck config matrix, EHDB-default-not-lock-in). Roadmap Phase 10 IMPLEMENTED + program status banner; _Sidebar link; Home + Sessions-Log entries.