send method
Implementation
@override
Future<SendResult> send(T value) async {
if (localSendChannel case final lc?) return lc.send(value);
if (isSendClosed) return const SendErrorDisconnected();
// Remote with Expando-based flow control
final t0 = mx.startSendTimer();
final rc = _ensureConnection();
await rc.send(value);
mx.sendOk(t0);
return const SendOk();
}