One retry step per side effect

// before: one step for the whole batch; a retry re-sends to everyone
await io.runTask("notify-all", () => Promise.all(recipients.map(send)));

// after: one step per recipient; finished steps are skipped on retry
for (const r of recipients) {
  await io.runTask(`notify-${r.id}`, async () => {
    if (!(await stillEligible(r.id, envelopeId))) return; // re-check at run time
    await send(r);
  });
}