Add Version property to StreamAssignment and ConsumerAssignment so future-version
entries from newer cluster peers are gracefully skipped rather than applied incorrectly.
ProcessStreamAssignment and ProcessConsumerAssignment now reject entries with
Version > CurrentVersion (1), incrementing SkippedUnsupportedEntries for operator
visibility. Add MetaEntryType.Unknown with a no-op ApplyEntry handler so unrecognised
RAFT entry types never crash the cluster. Version 0 is treated as current for
backward compatibility with pre-versioned assignments.
