Function set_candidate_status 

pub fn set_candidate_status(
    fs: &impl Fs,
    paths: &StorePaths,
    clock: &impl Clock,
    doc_id: &str,
    cand_id: &str,
    new_status: CandidateStatus,
) -> Result<CandidateEntry, SessionError>

Transition a candidate’s lifecycle status by appending a superseding entry (same id/page_id/snapshot_hash, new status + fresh timestamp). The scratch index stays append-only and auditable; list_scratch resolves the latest status per id via last-write-wins.

Returns SessionError if cand_id does not match any known candidate.