fix(alarm): make all date math wall-clock correct across DST and timezone changes

Full time-domain audit (three shipped date bugs prompted it) found one
root cause and two latent travel defects, all now fixed:

Day-stepping used add(Duration(days: 1)), which shifts the absolute
instant by exactly 86400s — documented Dart behavior (sdk#47666), so
crossing a DST transition drifted the wall hour by +-1h permanently
for the rest of the candidate scan (verified: 2026-03-28 07:30
Europe/Madrid + "1 day" = 08:30). The native Calendar engine preserves
wall time, and the single-authority fix made the drifted Dart verdict
win. Candidates now advance by calendar reconstruction (_siguienteDia:
DateTime(y, m, d+1, hora, minuto)), the same wall-clock-preserving
semantics as Calendar.add(DAY_OF_YEAR, 1) plus AOSP DeskClock's
defensive hour/minute re-assertion, keeping both engines in agreement
through any transition.

Instant-valued fields (snoozeHasta/snoozeOrigen/proximaEjecucion/
ultimaEjecucionGestionada/creadaEn/actualizadaEn) serialized as
offset-less local ISO, so re-parsing after a device timezone change
reinterpreted the same wall fields as a different instant. They now
serialize as UTC ("Z"); reads normalize to local, and legacy
offset-less payloads parse identically — no migration. fechaUnica
stays local on purpose: it is a wall-clock date.

One-shot alarms sent fechaUnica's midnight epoch to the native side,
whose boot/travel re-arm derives the calendar day back from it in the
CURRENT zone — a westward shift rolled the date to the previous day.
The channel now anchors the date at local noon, keeping it stable
across real-world zone shifts.

Property tests lock the no-drift guarantee (400 daily / 200 weekday
iterations must all land exactly at hora:minuto — on DST-observing
dev machines this crosses real transitions), plus UTC round-trip,
legacy-payload compatibility, and wall-date preservation tests.

lib/modelos/alarma_musical.dart

@@ -134,12 +134,19 @@ class AlarmaMusical {
     'volumen': volumen,
     'fadeInSegundos': fadeInSegundos,
     'sonidoInterno': sonidoInterno.name,
-    'proximaEjecucion': proximaEjecucion?.toIso8601String(),
-    'snoozeHasta': snoozeHasta?.toIso8601String(),
-    'snoozeOrigen': snoozeOrigen?.toIso8601String(),
-    'ultimaEjecucionGestionada': ultimaEjecucionGestionada?.toIso8601String(),
-    'creadaEn': creadaEn?.toIso8601String(),
-    'actualizadaEn': actualizadaEn?.toIso8601String(),
+    // INSTANT fields serialize as UTC (offset-carrying "Z" ISO): a local
+    // toIso8601String() has no offset, so re-parsing it after the device
+    // changes timezone reinterprets the same wall fields as a DIFFERENT
+    // instant (a snooze set in Madrid would shift hours after landing in
+    // New York). fechaUnica stays local-ISO on purpose: it is a wall-clock
+    // DATE (only y/m/d are ever read), which must follow the user.
+    'proximaEjecucion': proximaEjecucion?.toUtc().toIso8601String(),
+    'snoozeHasta': snoozeHasta?.toUtc().toIso8601String(),
+    'snoozeOrigen': snoozeOrigen?.toUtc().toIso8601String(),
+    'ultimaEjecucionGestionada':
+        ultimaEjecucionGestionada?.toUtc().toIso8601String(),
+    'creadaEn': creadaEn?.toUtc().toIso8601String(),
+    'actualizadaEn': actualizadaEn?.toUtc().toIso8601String(),
   };
 
   // persistence-resilience (D2): `id` stays a REQUIRED, un-defaulted cast
@@ -196,8 +203,12 @@ class AlarmaMusical {
     return Emisora.fromMap(Map<String, dynamic>.from(raw));
   }
 
+  // Normalizes to LOCAL on read: new payloads carry "Z" (UTC instants,
+  // toLocal converts), legacy offset-less payloads parse as local already
+  // (toLocal is then the identity) — both shapes land as the same local
+  // DateTime the scheduling math expects, so no data migration is needed.
   static DateTime? _dateFromJson(Object? raw) =>
-      raw is String ? DateTime.tryParse(raw) : null;
+      raw is String ? DateTime.tryParse(raw)?.toLocal() : null;
 
   static T _enumFromName<T extends Enum>(
     List<T> values,

lib/servicios/servicio_alarmas_android.dart

@@ -239,7 +239,20 @@ class ServicioAlarmasAndroid implements PuertoAlarmasAndroid {
       'minute': alarma.minuto,
       'scheduleType': alarma.tipoProgramacion.name,
       'weekdays': alarma.diasSemana,
-      'oneShotDateMillis': alarma.fechaUnica?.millisecondsSinceEpoch,
+      // Anchored at LOCAL NOON, not midnight: the native side derives the
+      // calendar DAY back from this epoch in whatever timezone the device is
+      // in when it re-arms (boot/travel). A midnight epoch reinterpreted in
+      // a westward zone rolls to the previous day; noon keeps the intended
+      // date stable across any real-world zone shift (+-11h).
+      'oneShotDateMillis':
+          alarma.fechaUnica == null
+              ? null
+              : DateTime(
+                alarma.fechaUnica!.year,
+                alarma.fechaUnica!.month,
+                alarma.fechaUnica!.day,
+                12,
+              ).millisecondsSinceEpoch,
       'snoozeUntilMillis': alarma.snoozeHasta?.millisecondsSinceEpoch,
       'snoozeOriginMillis': alarma.snoozeOrigen?.millisecondsSinceEpoch,
       'snoozeMinutes': alarma.snoozeMinutos,

lib/servicios/servicio_programacion_alarmas.dart

@@ -28,7 +28,7 @@ class ServicioProgramacionAlarmas {
             ? inicio
             : _sigueSiendoInminente(inicio, desde)
             ? inicio
-            : inicio.add(const Duration(days: 1));
+            : _siguienteDia(inicio, alarma);
 
     return switch (alarma.tipoProgramacion) {
       TipoProgramacionAlarma.unica =>
@@ -93,7 +93,7 @@ class ServicioProgramacionAlarmas {
           _esValida(alarma, actual, vacaciones, excepciones)) {
         return _normalizarInminente(actual, desde);
       }
-      actual = actual.add(const Duration(days: 1));
+      actual = _siguienteDia(actual, alarma);
     }
     return null;
   }
@@ -113,11 +113,28 @@ class ServicioProgramacionAlarmas {
           _esValida(alarma, actual, vacaciones, excepciones)) {
         return _normalizarInminente(actual, desde);
       }
-      actual = actual.add(const Duration(days: 1));
+      actual = _siguienteDia(actual, alarma);
     }
     return null;
   }
 
+  /// Advances a candidate to the SAME wall-clock time on the next calendar
+  /// day. `add(Duration(days: 1))` must never be used for this: Dart
+  /// Duration arithmetic shifts the absolute instant by exactly 86400s, so
+  /// crossing a DST transition drifts the wall hour by +-1h PERMANENTLY for
+  /// the rest of the scan (verified: 2026-03-28 07:30 Europe/Madrid +1d ->
+  /// 08:30). Calendar reconstruction preserves the alarm's wall-clock time
+  /// through any transition — the same semantics as the native engine's
+  /// Calendar.add(DAY_OF_YEAR, 1), keeping both sides in agreement. The
+  /// DateTime constructor normalizes day/month/year overflow.
+  DateTime _siguienteDia(DateTime actual, AlarmaMusical alarma) => DateTime(
+    actual.year,
+    actual.month,
+    actual.day + 1,
+    alarma.hora,
+    alarma.minuto,
+  );
+
   bool _esValida(
     AlarmaMusical alarma,
     DateTime candidato,

test/modelos/alarma_musical_fechas_test.dart

@@ -0,0 +1,70 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pluriwave/modelos/alarma_musical.dart';
+
+void main() {
+  AlarmaMusical base({DateTime? snoozeHasta, DateTime? fechaUnica}) =>
+      AlarmaMusical(
+        id: 'f1',
+        nombre: 'Fechas',
+        hora: 7,
+        minuto: 30,
+        tipoProgramacion:
+            fechaUnica != null
+                ? TipoProgramacionAlarma.unica
+                : TipoProgramacionAlarma.diaria,
+        diasSemana: const [],
+        fechaUnica: fechaUnica,
+        snoozeHasta: snoozeHasta,
+        snoozeOrigen: snoozeHasta?.subtract(const Duration(minutes: 5)),
+        ultimaEjecucionGestionada: snoozeHasta?.subtract(
+          const Duration(minutes: 5),
+        ),
+      );
+
+  test('los instantes se serializan como UTC con offset explicito (Z)', () {
+    // A local toIso8601String() carries no offset, so re-parsing it after a
+    // device timezone change reinterprets the same wall fields as a
+    // DIFFERENT instant (a snooze set in Madrid would shift hours after
+    // landing in New York). Instants must round-trip offset-carrying.
+    final snooze = DateTime(2026, 7, 13, 7, 10);
+    final json = base(snoozeHasta: snooze).toJson();
+
+    expect((json['snoozeHasta'] as String).endsWith('Z'), isTrue);
+    expect((json['snoozeOrigen'] as String).endsWith('Z'), isTrue);
+    expect(
+      (json['ultimaEjecucionGestionada'] as String).endsWith('Z'),
+      isTrue,
+    );
+  });
+
+  test('el round-trip de instantes preserva el instante exacto', () {
+    final snooze = DateTime(2026, 7, 13, 7, 10);
+    final recuperada = AlarmaMusical.fromJson(base(snoozeHasta: snooze).toJson());
+
+    expect(recuperada.snoozeHasta, snooze);
+    expect(
+      recuperada.snoozeOrigen,
+      snooze.subtract(const Duration(minutes: 5)),
+    );
+  });
+
+  test('los payloads legados sin offset siguen leyendose como hora local '
+      '(sin migracion)', () {
+    final legado = base().toJson()
+      ..['snoozeHasta'] = '2026-07-13T07:10:00.000';
+    final recuperada = AlarmaMusical.fromJson(legado);
+
+    expect(recuperada.snoozeHasta, DateTime(2026, 7, 13, 7, 10));
+  });
+
+  test('fechaUnica sigue siendo fecha de pared: conserva y/m/d en local', () {
+    final unica = DateTime(2026, 7, 13);
+    final recuperada = AlarmaMusical.fromJson(
+      base(fechaUnica: unica).toJson(),
+    );
+
+    expect(recuperada.fechaUnica!.year, 2026);
+    expect(recuperada.fechaUnica!.month, 7);
+    expect(recuperada.fechaUnica!.day, 13);
+  });
+}

test/servicios/servicio_programacion_alarmas_test.dart

@@ -167,6 +167,74 @@ void main() {
       expect(proxima, DateTime(2026, 5, 26, 7, 30));
     });
 
+    test('la hora de pared nunca deriva al iterar dias (DST-safe): diaria', () {
+      // Property lock for the DST drift bug: advancing candidates with
+      // add(Duration(days: 1)) shifts the ABSOLUTE instant by 86400s, so on
+      // DST-observing machines (this repo's dev machine runs Europe/Madrid)
+      // crossing 2026-03-29 / 2026-10-25 drifted the wall hour +-1h
+      // permanently. Iterating a full year of next-occurrences must keep
+      // every result at exactly hora:minuto.
+      final alarma = AlarmaMusical(
+        id: 'dst-diaria',
+        nombre: 'Diaria DST',
+        hora: 7,
+        minuto: 30,
+        tipoProgramacion: TipoProgramacionAlarma.diaria,
+        diasSemana: const [],
+      );
+
+      var proxima =
+          servicio.calcularProxima(
+            alarma: alarma,
+            desde: DateTime(2026, 1, 5, 6, 0),
+          )!;
+      for (var i = 0; i < 400; i++) {
+        expect(
+          proxima.hour,
+          7,
+          reason: 'deriva de hora de pared en ${proxima.toIso8601String()}',
+        );
+        expect(proxima.minute, 30);
+        proxima =
+            servicio.calcularSiguienteDespuesDeEjecucion(
+              alarma: alarma,
+              ejecucion: proxima,
+            )!;
+      }
+    });
+
+    test('la hora de pared nunca deriva al iterar dias (DST-safe): '
+        'dias de semana', () {
+      final alarma = AlarmaMusical(
+        id: 'dst-semana',
+        nombre: 'Semana DST',
+        hora: 7,
+        minuto: 30,
+        tipoProgramacion: TipoProgramacionAlarma.diasSemana,
+        diasSemana: const [1, 3, 5],
+      );
+
+      var proxima =
+          servicio.calcularProxima(
+            alarma: alarma,
+            desde: DateTime(2026, 1, 5, 6, 0),
+          )!;
+      for (var i = 0; i < 200; i++) {
+        expect(
+          proxima.hour,
+          7,
+          reason: 'deriva de hora de pared en ${proxima.toIso8601String()}',
+        );
+        expect(proxima.minute, 30);
+        expect(const [1, 3, 5], contains(proxima.weekday));
+        proxima =
+            servicio.calcularSiguienteDespuesDeEjecucion(
+              alarma: alarma,
+              ejecucion: proxima,
+            )!;
+      }
+    });
+
     test('mantiene la ocurrencia diaria cuyo disparo acaba de pasar', () {
       // Contract the native AlarmScheduler.computeNextTriggerMillis MUST
       // mirror: a trigger that just passed within toleranciaDisparoInminente