fix(alarm): stop a duplicate fire event from killing the active ring

The live eventosAlarma stream and the one-shot obtenerEventoInicial()
both read the same native fire event on cold start, so the same alarm
id can reach _mostrarAlarmaSonando twice within the same tick. The
second, duplicate delivery correctly detected an alarm was already
active and hit the "ignored" branch — but that branch unconditionally
called ocultarNotificacionAlarma, whose native handler
(dismissAlarmNotification) unconditionally stops
PluriWaveAlarmService for that id.

Confirmed via on-device logcat: the duplicate's stop landed ~180ms
after the ring-scoped media-volume override was captured and ~2.3s
before the real native-to-Flutter handoff, so flutterOwnsRing was
still false and the teardown backstop restored the device's original
volume immediately. The Flutter/radio player kept ringing regardless
(it starts independently of the native service), now anchored to
whatever volume the device happened to be at — explaining both
"ignores the configured ramp" and "plays at the device's own volume."

The ignored branch now only hides the notification when the duplicate
carries a genuinely different alarm id than the one already ringing;
a duplicate of the SAME ring's own event is now a pure no-op.

lib/app.dart

@@ -334,7 +334,18 @@ class _PaginaPrincipalState extends State<_PaginaPrincipal> {
       debugPrint(
         '[PluriWave][alarmas] alarma ignorada porque ya hay una activa id=${alarma.id} activa=$_alarmaSonandoId',
       );
-      await alarmas.android.ocultarNotificacionAlarma(alarma.id);
+      // A duplicate delivery of the SAME ring's own fire event (the live
+      // eventosAlarma stream and the one-shot obtenerEventoInicial() read
+      // the same native event and can both reach here) must be a no-op:
+      // ocultarNotificacionAlarma -> dismissAlarmNotification unconditionally
+      // stops PluriWaveAlarmService for that id on the native side, which
+      // would tear down the currently-ringing service and undo the
+      // ring-scoped media-volume override long before the real handoff.
+      // Only hide the notification when a genuinely DIFFERENT alarm fired
+      // while this one is active (single-ring-at-a-time by design).
+      if (alarma.id != _alarmaSonandoId) {
+        await alarmas.android.ocultarNotificacionAlarma(alarma.id);
+      }
       return;
     }