fix(alarm): fail-safe alarm system overhaul (SDD alarm-system-overhaul, slice A)

Root-cause fix for the unstoppable-alarm incident (alarm rang 15 minutes,
only uninstall silenced it) plus systematic hardening of every stop path.

Native (Kotlin):
- Verified stop: stopActiveAlarm now derives its result from the real
  post-teardown state (companion instance + synchronous stopEverything +
  activeRingingId check) instead of reporting unconditional success.
- Atomic teardown: every stop path (stop action, notification button,
  snooze, missed, onDestroy, startForeground failure) funnels through one
  stopEverything() covering audio, wakelock, notification, foreground
  state and firing-record cleanup; player.release() guarded.
- Bounded ringing: 10-minute auto-silence armed via AlarmManager fires a
  FIRED->MISSED transition with a localized missed-alarm notification;
  repeating alarms keep their native rearm, deleted alarms never produce
  ghost MISSED notifications.
- Durable firing record with onStartCommand re-validation (resurrection
  guard) and boot-time stale cleanup; firing records cleared on every
  refuse/mismatch/cancel path.
- New notification-only dismissal channel (dismissAlarmNotificationOnly)
  so UI-level dedup can never kill a live ring's audio.

Flutter (Dart):
- Stop/disable/edit/delete of a ringing alarm always attempt to silence
  it; on native-query failure the stop falls back toward silence via the
  id-scoped legacy stop.
- Verified-stop results surface failures: the ringing screen keeps
  dismiss-by-design on success, but on a verified failure it stays up
  with a persistent force-stop banner (guarded against double-dismiss)
  and auto-dismisses if the ring ends externally (missed/notification).
- Missed events sync alarm bookkeeping without opening the ringing UI.
- 4 new l10n keys translated across all 13 locales (ARB guard green).

550 tests green, analyzer clean. Reviewed in 3 adversarial 4-lens rounds
(2 deterministic + 1 refuter-corroborated critical fixed); formal
gentle-ai receipt waived by maintainer authorization (correction scope
legitimately exceeded the frozen genesis paths). On-device QA checklist
in openspec/changes/alarm-system-overhaul/tasks.md pending before
archive.

test/helpers/fakes_alarmas.dart

@@ -11,6 +11,7 @@ class FakePuertoAlarmasAndroid implements PuertoAlarmasAndroid {
   final canceladas = <String>[];
   final detenidas = <String>[];
   final ocultadas = <String>[];
+  final soloOcultadas = <String>[];
   final ejecucionesNativas = <EjecucionAlarmaNativa>[];
   final snoozesNativos = <EstadoSnoozeNativo>[];
   final _eventos = StreamController<EventoAlarmaAndroid>.broadcast();
@@ -22,6 +23,27 @@ class FakePuertoAlarmasAndroid implements PuertoAlarmasAndroid {
   /// could not otherwise produce.
   bool fallaProgramar = false;
 
+  /// Test-only failure switch: when true, [detenerSonidoActivo] reports an
+  /// unconfirmed/failed stop instead of a confirmed one.
+  bool fallaDetener = false;
+
+  /// Simulates [PluriWaveAlarmService.activeRingingId]: the id the fake
+  /// reports as currently ringing, or null if nothing rings.
+  String? alarmaSonandoIdValor;
+
+  /// Test-only failure switch (Finding 2, fail-toward-silence): when true,
+  /// [alarmaSonandoId] throws instead of returning a value, exercising the
+  /// [detenerSonidoNativo] fallback in `EstadoAlarmas._detenerSiEstaSonando`.
+  bool fallaConsultaSonando = false;
+
+  /// Every alarm id [detenerSonidoActivo] was invoked for, in call order.
+  final detencionesActivas = <String>[];
+
+  /// Test-only gate (RES-2 guard test): when set, [detenerSonidoActivo]
+  /// awaits it before resolving, letting tests exercise an overlapping
+  /// in-flight call.
+  Completer<void>? detenerActivoGate;
+
   /// Simulates a native -> Flutter `alarmFired` MethodChannel event.
   void emitirEvento(EventoAlarmaAndroid evento) => _eventos.add(evento);
 
@@ -49,11 +71,39 @@ class FakePuertoAlarmasAndroid implements PuertoAlarmasAndroid {
     detenidas.add(alarmaId);
   }
 
+  @override
+  Future<String?> alarmaSonandoId() async {
+    if (fallaConsultaSonando) {
+      throw StateError('fake alarmaSonandoId failure');
+    }
+    return alarmaSonandoIdValor;
+  }
+
+  @override
+  Future<ResultadoDetencion> detenerSonidoActivo() async {
+    detencionesActivas.add(alarmaSonandoIdValor ?? '');
+    final gate = detenerActivoGate;
+    if (gate != null) await gate.future;
+    if (fallaDetener) {
+      return const ResultadoDetencion(detenido: false, estabaSonando: true);
+    }
+    return ResultadoDetencion(
+      detenido: true,
+      estabaSonando: alarmaSonandoIdValor != null,
+      alarmaId: alarmaSonandoIdValor,
+    );
+  }
+
   @override
   Future<void> ocultarNotificacionAlarma(String alarmaId) async {
     ocultadas.add(alarmaId);
   }
 
+  @override
+  Future<void> ocultarSoloNotificacion(String alarmaId) async {
+    soloOcultadas.add(alarmaId);
+  }
+
   @override
   Future<DiagnosticoAlarmasAndroid> diagnostico() async =>
       DiagnosticoAlarmasAndroid(