The row type.
Owns validation and persistence. Return
{ ok: false, errors } to keep the row open with per-field messages; a
message is recorded under the __row__ key.
OptionalonSaved: (row: T) => void
Called with the server's canonical row when a commit
returns { ok: true, row }, so the caller can patch it into the page.
The edit state and its mutators.
Row-level editing against a draft copy.