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.

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);
+  });
+}