Stability checkpoint — ADR index refresh + audit artefact

Single MR landed since stable-v1.0.41 (svc !177 → 2871140) :

📚 Documentation
- ADR-0054 status flipped Scaffolded → Active in the README flat
  index. regen-adr-index.sh picks up the new status from the ADR
  file's frontmatter (was activated 2026-04-23 when GitLab
  Observability went live on the mirador1 group).
- Stability check audit committed (docs/audit/stability-2026-04-23-
  2310.md) : 🔴 1→0 (file length blocking cleared), 🟡 12→10
  (improving trend).

Post-merge main pipeline #772 green on SHA 2871140 (MR !177 merge).