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/pantallas/pantalla_alarma_sonando.dart

@@ -40,6 +40,7 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
   bool _fallbackActivo = false;
   bool _radioIntentada = false;
   bool _audioFlutterConfirmado = false;
+  bool _volumenMediaRestaurado = false;
 
   @override
   void initState() {
@@ -138,6 +139,23 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
     );
   }
 
+  /// Restores the ring-scoped `STREAM_MUSIC` override at most once per
+  /// screen instance (Requirement: Ring-scoped device-volume override,
+  /// Scenario "Restore is idempotent across double-exit paths"). Both
+  /// `_silenciarAudio` (dismiss/snooze) and `dispose` call this; the guard
+  /// here plus the idempotent Kotlin-side restore together keep any
+  /// exit-path ordering safe. Failures never propagate — a broken restore
+  /// must not block dismiss/snooze.
+  Future<void> _restaurarVolumenMediaUnaVez() async {
+    if (_volumenMediaRestaurado) return;
+    _volumenMediaRestaurado = true;
+    try {
+      await context.read<EstadoAlarmas>().android.restaurarVolumenMedia();
+    } catch (e) {
+      debugPrint('[PluriWave][alarmas] restaurar volumen media fallo: $e');
+    }
+  }
+
   /// Shared local-audio teardown for stop and snooze (Design 2.3): the Dart
   /// fallback player and fade timer MUST die before the alarm is re-programmed
   /// natively, otherwise the local fallback keeps looping after snooze.
@@ -210,6 +228,7 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
     } catch (e) {
       debugPrint('[PluriWave][alarmas] pausar radio fallo: $e');
     }
+    await _restaurarVolumenMediaUnaVez();
   }
 
   /// Dismisses the alarm screen safely in both live-app and dead-app states.
@@ -240,6 +259,7 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
     _fadeInTimer?.cancel();
     _estadoSub?.cancel();
     _fallbackPlayer.dispose();
+    unawaited(_restaurarVolumenMediaUnaVez());
     super.dispose();
   }