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.

lib/servicios/servicio_alarmas_android.dart

@@ -142,19 +142,6 @@ abstract class PuertoAlarmasAndroid {
   Future<bool> solicitarPermisoNotificaciones();
   Future<bool> solicitarPermisoPantallaCompleta();
   Future<bool> solicitarExencionBateria();
-  Future<void> confirmarAudioFlutter(String alarmaId);
-
-  /// Forces `STREAM_MUSIC` to the fixed audible reference level for the
-  /// duration of an alarm ring so the alarm cannot be silenced by a device
-  /// media volume of 0 (Requirement: Ring-scoped device-volume override).
-  /// [fraccion] is reserved for future tuning; the current native
-  /// implementation always targets the device max regardless of its value.
-  Future<void> forzarVolumenMediaParaAlarma(double fraccion);
-
-  /// Restores `STREAM_MUSIC` to the value captured by
-  /// [forzarVolumenMediaParaAlarma]. Idempotent: safe to call even when no
-  /// override is active or it was already restored.
-  Future<void> restaurarVolumenMedia();
 
   Future<DiagnosticoAlarmasAndroid> diagnostico();
   Future<EventoAlarmaAndroid?> obtenerEventoInicial();
@@ -290,18 +277,6 @@ class ServicioAlarmasAndroid implements PuertoAlarmasAndroid {
   Future<void> detenerSonidoNativo(String alarmaId) =>
       _logAndInvokeVoid('stopNativeAlarmSound', {'id': alarmaId});
 
-  @override
-  Future<void> confirmarAudioFlutter(String alarmaId) =>
-      _logAndInvokeVoid('confirmFlutterAudio', {'id': alarmaId});
-
-  @override
-  Future<void> forzarVolumenMediaParaAlarma(double fraccion) =>
-      _logAndInvokeVoid('overrideMediaVolumeForRing', {'fraction': fraccion});
-
-  @override
-  Future<void> restaurarVolumenMedia() =>
-      _logAndInvokeVoid('restoreMediaVolume', {});
-
   @override
   Future<bool> solicitarPermisoAlarmasExactas() async {
     final abierto = await _channel.invokeMethod<bool>(