Migration — sequencing and cutover

The irreversible part. Everything else in this KB can be revised later; the wipe cannot.

Cutover runbook — ordered sequence, pre-wipe through first provisioned boot.
Backup posture — what restic holds, and two exclusions that are currently wrong.

KB index — parent: full registry.
How the collection gets verified — prerequisite: the untested path runs here for the first time.