ChunkVersion::operator== considers identical UNTRACKED, IGNORED, and {epoch,timestamp,0,0} versions equal, but operator<=> currently returns std::partial_ordering::unordered for those same pairs.

This creates inconsistent comparison semantics and can cause callers that use the three-way comparison to treat equal shard placement versions as non-comparable, triggering pessimistic stale/recovery handling. Identical versions should compare as equivalent; only genuinely non-comparable version states should return unordered.