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/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,