Function import_once 

pub(crate) fn import_once(
    fs: &FileStore,
    uid: &str,
    stores: Option<&[PathBuf]>,
    disk_sha: impl Fn(&str) -> Option<String>,
    deadline: Instant,
) -> Result<Option<usize>, StoreError>

Import uid’s earlier history, once, from stores (every history store on the machine when None). disk_sha gives the content hash of a file in the agent’s proven folder. Returns how many versions were imported, or None when there was nothing to do (already imported, or out of time — retried next pass).