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.

test/pantallas/pantalla_alarma_sonando_test.dart

@@ -31,6 +31,7 @@ class _Entorno {
 Future<_Entorno> _montarPantalla(
   WidgetTester tester, {
   int snoozeMinutos = 5,
+  int fadeInSegundos = 0,
   // Slice 3 (fade-in dedup): existing callers rely on the radio already
   // being "reproduciendo" by mount time, which cancels the fallback timer
   // synchronously and leaves nothing to observe mid-handoff. Fade-in-gate
@@ -75,6 +76,7 @@ Future<_Entorno> _montarPantalla(
       tipoProgramacion: TipoProgramacionAlarma.diaria,
       diasSemana: const [],
       snoozeMinutos: snoozeMinutos,
+      fadeInSegundos: fadeInSegundos,
       emisora: const Emisora(
         uuid: 'e1',
         nombre: 'Radio Uno',
@@ -255,6 +257,52 @@ void main() {
     });
   });
 
+  group('salidas del ring fuera de los botones', () {
+    testWidgets('el boton atras del sistema se comporta como Detener: para la '
+        'radio, finaliza y cierra', (tester) async {
+      final entorno = await _montarPantalla(tester);
+
+      // Simulate the SYSTEM back (routes through PopScope via the binding),
+      // not a direct Navigator.pop: a plain pop only runs dispose(), which
+      // leaves the shared radio player ringing with no alarm UI left to
+      // stop it.
+      await tester.binding.handlePopRoute();
+      await tester.pumpAndSettle();
+
+      expect(find.byType(PantallaAlarmaSonando), findsNothing);
+      expect(
+        entorno.audio.pausas,
+        greaterThanOrEqualTo(1),
+        reason: 'atras debe pausar la radio como lo hace Detener',
+      );
+      expect(
+        entorno.android.ocultadas,
+        contains('ring1'),
+        reason: 'atras debe finalizar la ejecucion (mismo camino que Detener)',
+      );
+    });
+
+    testWidgets('salir a mitad de rampa restaura la ganancia del reproductor '
+        'a 1.0 para la radio normal', (tester) async {
+      final entorno = await _montarPantalla(tester, fadeInSegundos: 30);
+
+      // Let the ramp advance a few steps (250ms per step) so the shared
+      // handler gain sits at a partial value well below 1.0.
+      await tester.pump(const Duration(seconds: 2));
+      expect(entorno.audio.volumenesAplicados.last, lessThan(0.2));
+
+      await tester.tap(find.text(l10n.stopAlarmAction));
+      await tester.pumpAndSettle();
+
+      expect(
+        entorno.audio.volumenesAplicados.last,
+        1.0,
+        reason: 'la salida del ring no debe dejar la radio normal al nivel '
+            'parcial de la rampa',
+      );
+    });
+  });
+
   group('restore de volumen de medios con dispose como unico llamador', () {
     testWidgets('desmontar la pantalla sin detener ni posponer restaura el '
         'volumen exactamente una vez', (tester) async {