refactor(alarm): make the ringing screen pure UI over a reduced native port

PantallaAlarmaSonando no longer owns any audio orchestration (fallback
player, dB ramp, native handoff confirm, media-volume override/restore):
it only calls EstadoAlarmas.finalizarEjecucion/posponerAlarma from
Stop/Snooze/back, keeping the single-exit guard, PopScope back=Stop and
dismiss semantics intact. The status line now reads directly from the
alarm's static config (station name or a neutral label) instead of a
live playback/handoff state.

PuertoAlarmasAndroid drops confirmarAudioFlutter,
forzarVolumenMediaParaAlarma and restaurarVolumenMedia, and app.dart no
longer pre-starts a station before pushing the ring screen. This is the
Dart half of moving to a single native ring-audio owner (WU1 of 2); the
Kotlin service rebuild lands next and keeps this intermediate state
shippable with no double audio.

test/servicios/servicio_alarmas_android_test.dart

@@ -107,31 +107,4 @@ void main() {
     },
   );
 
-  test(
-    'forzarVolumenMediaParaAlarma invoca overrideMediaVolumeForRing con fraction',
-    () async {
-      final servicio = ServicioAlarmasAndroid(channel: channel);
-
-      await servicio.forzarVolumenMediaParaAlarma(1.0);
-
-      final llamada = llamadas.singleWhere(
-        (c) => c.method == 'overrideMediaVolumeForRing',
-      );
-      expect(llamada.arguments, {'fraction': 1.0});
-    },
-  );
-
-  test(
-    'restaurarVolumenMedia invoca restoreMediaVolume sin argumentos',
-    () async {
-      final servicio = ServicioAlarmasAndroid(channel: channel);
-
-      await servicio.restaurarVolumenMedia();
-
-      final llamada = llamadas.singleWhere(
-        (c) => c.method == 'restoreMediaVolume',
-      );
-      expect(llamada.arguments, <String, Object?>{});
-    },
-  );
 }