Per mara's review: validate_credential_name allowed any [A-Za-z0-9_.-]
byte sequence, which permits a literal ".." substring. Not currently
exploitable (snapshot_path() embeds the label inside a single
format!()'d path component with no "/" in the allowed charset, so
there's no directory to traverse into), but it's a landmine for any
future caller that builds a path via PathBuf::from(name) directly
instead of the current string-embedding. Reject ".." outright in the
shared validator, plus a matching client-side check in hivectl for
fail-fast UX (hive-priv's copy is still the authoritative one).
720ac81235