temperature: double, readable
relay: boolean, readable + writable

auto temperature = device.datastream<double>("temperature");
auto relay = device.datastream<bool>("relay");

relay.onWrite([](bool enabled) {
  if (!safeToSwitch()) return FlovaWriteResult::reject("interlock_open");
  digitalWrite(RELAY_PIN, enabled ? HIGH : LOW);
  return FlovaWriteResult::accept();
});

temperature.report(readTemperature());