Add PressureState() and writer wait tracking to WALAdmission, WALStatus
snapshot API on BlockVol, WAL sizing guidance pure functions, Prometheus
histogram/gauge/counter exports, and admin /status WAL fields. 23 new
tests (7 admission, 10 guidance, 6 QA adversarial).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>