feat(alarm): make the ring immune to device media volume

The alarm's steady-state audio runs on the Flutter media-stream
player after the native handoff, so device volume 0 silenced it
entirely. The ring now forces STREAM_MUSIC to an audible reference:
Dart requests the override before pre-starting alarm audio (fallback
WAV included), Kotlin captures the current volume once and restores
it idempotently on every exit path (dismiss, snooze, dispose), with
a native best-effort backstop in service teardown.

The backstop is handoff-aware via PluriWaveAlarmService.flutterOwnsRing:
confirmFlutterAudio marks the handoff before triggering the native
stop, so the backstop cannot restore the volume mid-ring right as the
Flutter player takes over (that would re-silence the alarm at volume
0). The flag resets at every ring start; Flutter process death after
handoff remains a documented best-effort gap.

The alarm's perceived loudness keeps ramping 5% to the configured
volume through the player as before; normal radio playback and call
ducking never touch the override.

Work unit 2/3 of alarm-volume-ramp-restore (ring volume override).

lib/servicios/servicio_alarmas_android.dart

@@ -143,6 +143,19 @@ abstract class PuertoAlarmasAndroid {
   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();
   Future<List<EjecucionAlarmaNativa>> obtenerEjecucionesNativasGestionadas();
@@ -281,6 +294,14 @@ class ServicioAlarmasAndroid implements PuertoAlarmasAndroid {
   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>(