instance_identity is one row made by migration 0127 with a random id, so an
instance has it before its first account. A full backup carries it in its
header as instance_id (the tar manifest header and the JSON top level);
BACKUP_VERSION is 28. The table rides in _NOT_INCLUDED: a restore decides from
the header value rather than copying a row.
Two stores with the same id number their rows the same way. That is what lets
a later step roll a live instance back by original id, refuse a backup whose
ids would land on unrelated rows, and have a fresh-instance restore adopt the
backup's id (adopt_instance_id).
Milestone 582, step 1.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
