Function load_progress 

pub fn load_progress(
    workspace: &Path,
    session_id: &str,
) -> Result<Option<ProgressLedger>, SessionStoreError>

Load the progress ledger for a session, if one has been persisted.

Returns Ok(None) when no ledger file exists yet (a fresh or pre-ledger session) rather than an error, so callers can treat absence as “no signal”.