docs(m12): refresh API, specs, ops, and roadmap to the shipped M12 reality
- API.md: document `similar_to`/`region`/`unavailable_shards` on /feed and
/search, the new POST /vector_search k-NN probe, and the cluster-node-only
routes (/cluster/*, /sharded/*, /hardnegs)
- CHANGELOG.md: M12 entries — multi-vector preference + ANN candidate-gen,
idle-readiness + TLS scale-up (m12p5/p6), sharded ingestion (m12p4)
- ROADMAP.md: mark M11 + M12 COMPLETE; restate the v1.0 bar (30-day-green
nightly calendar + Ref-A/k3s throughput re-runs)
- prometheus-alerts.yaml: add ship-stall, quorum-lag, divergence-quarantine,
reseed-pending, and snapshot-pin-force-drop cluster alerts
- check-docs.sh: self-updating milestone-status freshness guard derived from
ROADMAP's latest COMPLETE milestone
- refresh specs (00-14), ai-lookup, guides, and runbooks to M0-M12
4051077cff