feat(android): dual prefetch delay, native fetcher, scoped content cache

- Schedule dual content fetch with WorkManager initialDelay to the next
  contentFetch cron; reschedule from prefs after success and on boot when
  dual_fetch_* exists (DualScheduleFetchRecovery + ReactivationManager).
- When contentFetch has no URL, call NativeNotificationContentFetcher with
  FetchContext (prefetch + next notify time); else keep HTTP/mock behavior.
- Add content_cache.cacheScope (dual|daily|legacy), Room v4 migration,
  getLatestByScope; DualScheduleHelper reads dual only; daily fetch paths
  write daily; NotifyReceiver prefers daily/legacy for legacy cache reads.
- Extract ScheduleCronUtils.calculateNextRunTimeMillis for shared cron math.
- Document in README/CHANGELOG; bump package to 2.1.5.

doc/platform/android/ANDROID_DUAL_SCHEDULE_NATIVE_FETCH_AND_CACHE_SCOPE.md

@@ -1,6 +1,6 @@
 # Android dual schedule: native fetch, WorkManager timing, and scoped content cache
 
-**Status:** Draft — implementation plan (pre-code)  
+**Status:** Implemented (Android) — see CHANGELOG [2.1.5]  
 **Date:** 2026-03-25  
 **Scope:** `daily-notification-plugin` Android (Kotlin/Java), dual / “New Activity” schedule (`scheduleDualNotification`)