fix(alarm): defer the Dart fade-in until the native handoff confirms

The native service and the Flutter player each ran their own 5%-to-
target fade-in, and both could drive audible volume at the handoff,
producing a jump or ramp reset. The Dart ramp now starts exactly once
from the handoff-confirmation path: the player still pre-starts at 5%,
and _confirmarAudioFlutterListo() starts the ramp in a finally block
so it runs whether the native confirmation succeeds or fails — the
alarm can never stay stuck at 5% if the native side is already gone.

Work unit 3/3 of alarm-volume-ramp-restore (fade-in dedup).

lib/pantallas/pantalla_alarma_sonando.dart

@@ -64,7 +64,6 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
     if (!widget.audioPrearrancado) {
       unawaited(radio.reproducir(emisora));
     }
-    _iniciarFadeIn();
 
     // S7-R4 boundary: only `reproduciendo` cancels the fallback timer —
     // `reconectando`/`cargando` do NOT count as playing, so the 12-second
@@ -94,7 +93,6 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
     _fallbackActivo = true;
     await _fallbackPlayer.setAsset(_assetFallback(widget.alarma.sonidoInterno));
     await _fallbackPlayer.play();
-    _iniciarFadeIn();
     await _confirmarAudioFlutterListo();
     if (mounted) setState(() {});
   }
@@ -131,12 +129,28 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
     await _fallbackPlayer.setVolume(volumen.clamp(0.0, 1.0));
   }
 
+  /// Confirms the native-to-Flutter audio handoff at most once per screen
+  /// instance, then starts the single audible Dart fade-in ramp (Slice 3:
+  /// fade-in dedup at handoff). The native ramp
+  /// (`PluriWaveAlarmService.startFadeIn`) owns audio until this
+  /// confirmation lands; starting the Dart ramp any earlier would
+  /// interleave both ramps and produce an audible jump/reset. If the
+  /// native confirmation channel call fails (dead or never-there native
+  /// side), the fade-in still starts in the `finally` block below — Dart
+  /// is the only audible source either way, so the ring must never stay
+  /// stuck at [_volumenInicialFadeIn] forever.
   Future<void> _confirmarAudioFlutterListo() async {
     if (_audioFlutterConfirmado) return;
     _audioFlutterConfirmado = true;
-    await context.read<EstadoAlarmas>().android.confirmarAudioFlutter(
-      widget.alarma.id,
-    );
+    try {
+      await context.read<EstadoAlarmas>().android.confirmarAudioFlutter(
+        widget.alarma.id,
+      );
+    } catch (e) {
+      debugPrint('[PluriWave][alarmas] confirmar audio flutter fallo: $e');
+    } finally {
+      _iniciarFadeIn();
+    }
   }
 
   /// Restores the ring-scoped `STREAM_MUSIC` override at most once per