saveSnapshot

Replaces the snapshot and atomically drops every appended change and any stored meta. This is compaction: the new snapshot already contains those changes, so keeping them would replay them twice, and it embeds a newer header than meta holds — a snapshot envelope embeds its own checkpoint and changeID, newer than whatever meta held; applying the old header over the new snapshot would regress serverSeq and — worse — lamport, whose regression makes the next edit mint tickets that collide with identities already in the restored root.