appendChange

Appends one local change. This is the hot path — frequent and small — so an implementation must not rewrite the whole entry to satisfy it.

It is an upsert keyed by StoredChange.clientSeq: re-appending a change already stored replaces it rather than duplicating it, so a retried write is safe. It is a silent no-op on a key with no entry — a row written anyway is an orphan load cannot see.