Function migrate_data_dir_to 

pub async fn migrate_data_dir_to(
    source_dir: &Path,
    destination_dir: &Path,
    destination_engine: StorageEngine,
) -> Result<StoreMigrationReceipt, PortError>

migrate_data_dir with the destination engine chosen — how a store changes engines (ADR-018). A redb store becomes a SQLite one that two agent hosts can share by replaying its history into a fresh directory; the source stays as it was and the receipt records both layouts.