ClientId

Opaque, comparable client identity for Raft §8 client-serial dedup.

The auto form (auto) is "auto:$nodeId-$randomHex": a reserved auto: sentinel, the cluster-unique NodeId, and a per-incarnation random hex suffix. A caller that wants cross-crash exactly-once passes any stable opaque string it persists itself (and must not use the reserved auto: namespace). Raft-the-transport never parses the value; the one structural reader is autoFamily, localized here because this type owns the format.