fix(alarmas): wire scheduling failures into per-alarm exceptions

guardarAlarma/posponerAlarma/posponerProximaDesdePreaviso now record
a scheduling failure via ServicioAlarmas.registrarFalloProgramacion
on catch and clear it on a successful (re)schedule, in addition to
the existing transient EstadoAlarmas.error string. This makes the
failure visible per-alarm via ultimaExcepcionPara instead of only a
generic app-wide message.

Also fixes _sincronizarTodas: a single alarm's android.programar
throw used to abort the whole loop, silently skipping every sibling
alarm scheduled AFTER it on that pass (including on every app launch,
via inicializar). Each alarm's outcome is now independent.

test/helpers/fakes_alarmas.dart

@@ -41,6 +41,12 @@ class FakePuertoAlarmasAndroid implements PuertoAlarmasAndroid {
   /// could not otherwise produce.
   bool fallaProgramar = false;
 
+  /// Test-only PER-ALARM failure switch (fix/alarmas-fallos-silenciosos):
+  /// [programar] throws only for ids in this set, letting a test simulate
+  /// one alarm failing to schedule while its siblings succeed -- the global
+  /// [fallaProgramar] switch cannot express that (it fails everything).
+  final Set<String> idsFallanProgramar = {};
+
   /// Test-only failure switch: when true, [detenerSonidoActivo] reports an
   /// unconfirmed/failed stop instead of a confirmed one.
   bool fallaDetener = false;
@@ -73,7 +79,7 @@ class FakePuertoAlarmasAndroid implements PuertoAlarmasAndroid {
 
   @override
   Future<void> programar(AlarmaMusical alarma) async {
-    if (fallaProgramar) {
+    if (fallaProgramar || idsFallanProgramar.contains(alarma.id)) {
       throw StateError('fake programar failure');
     }
     programadas.add(alarma);