feat(alarms): native reliability fixes and end-to-end snooze

- Use mediaPlayback|systemExempted FGS type with FOREGROUND_SERVICE_SYSTEM_EXEMPTED so alarms fire on Android 14+ (FOREGROUND_SERVICE_ALARM does not exist in the SDK)
- Deduplicate fire notifications: the foreground service FSI notification is the single owner; receiver path removed
- Notification channel v2 with alarm sound URI and USAGE_ALARM attributes, one-time guarded migration from legacy channels
- Pass fallback station through the MethodChannel (NativeAlarmSpec schemaVersion 3) with a three-stage audio chain: primary -> fallback station -> bundled WAV
- Native fade-in volume ramp honoring fadeInSegundos when the app is killed
- Request battery-optimization exemption once, tracked with a persisted asked-once flag
- Fix snooze end-to-end: native ACTION_SNOOZE now reports back to Flutter (snoozed event + cold-start sync), snooze anchor unified to occurrence+minutes on both sides, periodic recalc no longer erases an active snooze
- Add snooze buttons (3/5/10/custom) to the ringing screen with shared audio teardown
- Redesign ringing screen on PluriWaveScaffold with reduced-motion-aware entry animation (new PluriAnimate helper)
- Alarm editor: live next-trigger preview, searchable station pickers (primary and fallback), configurable snooze duration, volume floor down to 0
- New alarm strings localized across all 13 locales
- New unit/widget tests for the snooze flow, alarm bridge payloads, ringing screen and editor (77 tests green)
- SDD artifacts for the app-quality-and-native-alarms change (explore, proposal, spec, design, tasks, apply progress)

lib/modelos/alarma_musical.dart

@@ -62,7 +62,9 @@ class AlarmaMusical {
     DateTime? fechaUnica,
     bool limpiarFechaUnica = false,
     Emisora? emisora,
+    bool limpiarEmisora = false,
     Emisora? emisoraFallback,
+    bool limpiarEmisoraFallback = false,
     bool? sonarEnVacaciones,
     int? snoozeMinutos,
     double? volumen,
@@ -87,8 +89,11 @@ class AlarmaMusical {
       tipoProgramacion: tipoProgramacion ?? this.tipoProgramacion,
       diasSemana: diasSemana ?? this.diasSemana,
       fechaUnica: limpiarFechaUnica ? null : fechaUnica ?? this.fechaUnica,
-      emisora: emisora ?? this.emisora,
-      emisoraFallback: emisoraFallback ?? this.emisoraFallback,
+      emisora: limpiarEmisora ? emisora : emisora ?? this.emisora,
+      emisoraFallback:
+          limpiarEmisoraFallback
+              ? emisoraFallback
+              : emisoraFallback ?? this.emisoraFallback,
       sonarEnVacaciones: sonarEnVacaciones ?? this.sonarEnVacaciones,
       snoozeMinutos: snoozeMinutos ?? this.snoozeMinutos,
       volumen: volumen ?? this.volumen,
@@ -98,7 +103,8 @@ class AlarmaMusical {
           limpiarProximaEjecucion
               ? proximaEjecucion
               : proximaEjecucion ?? this.proximaEjecucion,
-      snoozeHasta: limpiarSnooze ? snoozeHasta : snoozeHasta ?? this.snoozeHasta,
+      snoozeHasta:
+          limpiarSnooze ? snoozeHasta : snoozeHasta ?? this.snoozeHasta,
       snoozeOrigen:
           limpiarSnooze ? snoozeOrigen : snoozeOrigen ?? this.snoozeOrigen,
       ultimaEjecucionGestionada: