RepositoryExportDestination / RepositoryImportSource implement the
public OCP\UserMigration interfaces on top of the repository, so
user_migration's migrators (calendar, contacts, trashbin, account,
files) export straight into the encrypted repository.
Spike E (NC34): export 0.8s, unchanged second export stores nothing;
after deleting the user, import restores account, files (bit-identical),
calendar event and contact. Findings: calendars/address books come back
as 'migrated-*' with new object URIs; importing over an existing user
duplicates contacts and adds a migrated birthday calendar; creating the
user on import needs >128 MB (password_policy word list).
