feat(alarmas): verify native registration after a successful save

android.programar() returning without throwing was treated as proof
the OS registered the alarm -- this is exactly the gap the reported
case fell through. guardarAlarma now cross-checks a fresh native
pending-alarm count against how many alarms Dart believes are
active-with-a-next-run right after a successful schedule call, and
records a failure for the just-saved alarm when the native count
falls short.

FakePuertoAlarmasAndroid.alarmasNativasPendientes now defaults to a
count derived from programar()/cancelar() calls (mirroring the real
native scheduler's own registry) instead of a frozen 0, while any
test that explicitly assigns the field keeps getting exactly that
value regardless of what programar/cancelar do afterward -- verified
against the full suite, no regressions.

test/helpers/fakes_alarmas.dart

@@ -26,10 +26,27 @@ class FakePuertoAlarmasAndroid implements PuertoAlarmasAndroid {
   bool puedeProgramarExactas = true;
   bool notificacionesPermitidas = true;
   bool puedeUsarPantallaCompleta = true;
-  int alarmasNativasPendientes = 0;
   String fabricante = 'test';
   int versionSdk = 35;
 
+  /// Ids [programar] most recently scheduled as active-with-a-next-run (kept
+  /// in sync with [cancelar] too), mirroring the real native scheduler's own
+  /// pending-alarm registry (fix/alarmas-fallos-silenciosos, item 3: "verify
+  /// the alarm is actually registered"). Backs [alarmasNativasPendientes]'s
+  /// DEFAULT so a test that never touches that field gets a value that
+  /// tracks reality instead of a frozen `0` -- a test that explicitly
+  /// assigns the field (many `pantalla_diagnostico_alarmas_test.dart` cases
+  /// do, to model a stale/corrupt native count on purpose) keeps getting
+  /// EXACTLY that value regardless of what programar/cancelar do afterward.
+  final _idsRegistradosNativamente = <String>{};
+  int? _alarmasNativasPendientesFijado;
+
+  int get alarmasNativasPendientes =>
+      _alarmasNativasPendientesFijado ?? _idsRegistradosNativamente.length;
+
+  set alarmasNativasPendientes(int valor) =>
+      _alarmasNativasPendientesFijado = valor;
+
   /// Test-only failure switch (diagnostics screen, "intent not resolving"
   /// coverage): when true, every `abrir*`/`solicitar*` system-screen action
   /// below reports failure (as a real device does when a ROM lacks that
@@ -83,11 +100,17 @@ class FakePuertoAlarmasAndroid implements PuertoAlarmasAndroid {
       throw StateError('fake programar failure');
     }
     programadas.add(alarma);
+    if (alarma.activa && alarma.proximaProgramable != null) {
+      _idsRegistradosNativamente.add(alarma.id);
+    } else {
+      _idsRegistradosNativamente.remove(alarma.id);
+    }
   }
 
   @override
   Future<void> cancelar(String alarmaId) async {
     canceladas.add(alarmaId);
+    _idsRegistradosNativamente.remove(alarmaId);
   }
 
   @override