fix(alarm): scope native stop to the ringing id and intercept system back

Two exit-path holes found by adversarial review before the next build:

PluriWaveAlarmService.stopAlarm() never compared the requested id to
activeAlarmId, so any stop request for a DIFFERENT alarm tore down
whichever ring was active: with two alarms firing close together, the
second one's routine hide-notification call (via dismissAlarmNotification
-> ACTION_STOP) killed the first alarm mid-ring and prematurely restored
the device volume override. A mismatched id now only cancels that id's
notification and returns; null keeps full-teardown semantics for
internal/onDestroy callers.

The ringing screen never intercepted the system back gesture: a plain
route pop ran only dispose(), leaving the shared radio player ringing
with no alarm UI left anywhere to stop it. Back now routes through
PopScope into the same _detener() flow as the Stop button, guarded by a
single-exit flag so a back-press racing a button tap cannot run the
teardown twice and pop the route underneath.

Also resets the shared handler gain to 1.0 on ring exit: the fade-in
mutates the radio player's persistent volume, and exiting mid-ramp used
to leave every later radio play at the partial ramp level.

lib/pantallas/pantalla_alarma_sonando.dart

@@ -13,6 +13,7 @@ import '../modelos/alarma_musical.dart';
 import '../servicios/servicio_audio.dart';
 import '../tema/pluri_animate.dart';
 import '../tema/pluriwave_theme.dart';
+import '../tema/pluriwave_tokens.dart';
 import '../widgets/pluri_glass_surface.dart';
 import '../widgets/pluri_wave_scaffold.dart';
 
@@ -205,7 +206,15 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
     await _fallbackPlayer.stop();
   }
 
+  /// Single-exit guard: Stop, snooze and the system back gesture all funnel
+  /// into the same teardown; whichever lands first wins and the rest no-op,
+  /// so a back-press racing a button tap can never run the exit flow twice
+  /// (a second _dismissScreen would pop the route UNDER the alarm screen).
+  bool _salidaEnCurso = false;
+
   Future<void> _detener() async {
+    if (_salidaEnCurso) return;
+    _salidaEnCurso = true;
     final radio = context.read<EstadoRadio>();
     final alarmas = context.read<EstadoAlarmas>();
     await _silenciarAudio(radio);
@@ -225,6 +234,8 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
   /// through the canonical EstadoAlarmas.posponerAlarma, which hides the
   /// native notification (same stop path as dismiss) and re-programs Android.
   Future<void> _posponer(int minutos) async {
+    if (_salidaEnCurso) return;
+    _salidaEnCurso = true;
     final radio = context.read<EstadoRadio>();
     final alarmas = context.read<EstadoAlarmas>();
     // Captured BEFORE dismiss (Design D4): the ringing screen dismisses by
@@ -264,6 +275,15 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
     } catch (e) {
       debugPrint('[PluriWave][alarmas] pausar radio fallo: $e');
     }
+    try {
+      // The fade-in mutates the SHARED radio handler gain; exiting mid-ramp
+      // would otherwise leave every later radio play at the partial ramp
+      // level until the next full ramp or app restart. 1.0 is the handler's
+      // default gain; the player is already paused, so this is inaudible.
+      await radio.audio.setVolumen(1.0);
+    } catch (e) {
+      debugPrint('[PluriWave][alarmas] restaurar ganancia radio fallo: $e');
+    }
     await _restaurarVolumenMediaUnaVez();
   }
 
@@ -308,6 +328,25 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
     // the first frame after a screen-off FSI wake can stutter. The blur sigma
     // is capped here, and reduced-motion users skip the entry animation
     // entirely via pluriFadeIn.
+    return PopScope(
+      // System back / predictive back must behave exactly like Stop: a plain
+      // route pop would run only dispose(), leaving the shared radio player
+      // ringing with no alarm UI left anywhere to stop it.
+      canPop: false,
+      onPopInvokedWithResult: (didPop, _) {
+        if (didPop) return;
+        unawaited(_detener());
+      },
+      child: _cuerpo(context, alarma, l10n, tokens),
+    );
+  }
+
+  Widget _cuerpo(
+    BuildContext context,
+    AlarmaMusical alarma,
+    AppLocalizations l10n,
+    PluriWaveTokens tokens,
+  ) {
     return PluriWaveScaffold(
       body: SafeArea(
         child: Padding(