`SetupRecord.as_payload()` serialised the record verbatim, so the broadcast
nested `failure: {...}` while `setup.status` spread the same four keys flat.
A client keyed on `error_code` saw it on one surface and not the other. Flatten
it in `as_payload`, declare the three optional keys on `SetupReadyPayload`, and
regenerate the TS/OpenRPC contract.