Function maybe_snapshot_pre_migration 

pub fn maybe_snapshot_pre_migration(
    db_dir: &Path,
    snapshots_dir: &Path,
    channel: &str,
    code_version: &str,
    current_schema_version: i64,
) -> Result<Option<PathBuf>, SnapshotError>

High-level entry: check, snapshot, prune. Returns the snapshot dir on success, or None if no snapshot was needed. Errors are surfaced to the caller; production call sites should log-and-continue rather than abort boot (see module docs).