Adds IAlarmSubscribableConnection to MxGatewayDataConnection (shared session-less
feed, ref-counted), IMxGatewayClient.RunAlarmStreamAsync over the package
StreamAlarmsAsync with internal reconnect, and MxGatewayAlarmMapper
(AlarmFeedMessage/OnAlarmTransitionEvent -> NativeAlarmTransition). Behavior
verified against a live gateway in Task 28; mapper unit-tested.