The two shapes, if you drive it yourself

Every action returns one of two shapes, and the server picks which:

let res = await turbo.createArNSAction('buy-name', { name, ownerAddress });
if (res.status === 'awaiting-signature') {
  res = await turbo.signArNSAction(
    res.nonce,
    await owner.signTransaction(res.transaction),
  );
}
// res.status === 'completed'; res.messageId is the on-chain write

Branch on status, never on which action you called: setArNSRecord completes alone while Turbo is a controller and flips to awaiting-signature the moment you revoke Turbo. It degrades instead of breaking.

Sign the exact bytes returned. Turbo has already signed as fee payer; rebuilding the transaction invalidates that signature.