fix(alarm): capture alarm state before dispose so its volume restore works

_restaurarVolumenMediaUnaVez() read the BuildContext to reach the
alarm port, but dispose() runs after the element is defunct, so the
lookup always threw (caught and logged) and the dispose safety-net
never actually restored the media volume when it was the sole exit
path. The state is now captured once in initState and the restore
helper uses the field. Adds the missing dispose-as-sole-caller
regression test.

test/pantallas/pantalla_alarma_sonando_test.dart

@@ -228,4 +228,20 @@ void main() {
       expect(entorno.audio.volumenesAplicados, [0.05, 0.85]);
     });
   });
+
+  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 {
+      final entorno = await _montarPantalla(tester);
+
+      // Teardown that bypasses _detener()/_posponer() entirely: dispose()
+      // must work as a restore path on its own. Reading the BuildContext
+      // inside dispose() throws (the element is already defunct), so the
+      // port reference has to be captured while the widget is mounted.
+      await tester.pumpWidget(const SizedBox.shrink());
+      await tester.pumpAndSettle();
+
+      expect(entorno.android.volumenRestaurado, 1);
+    });
+  });
 }