Root cause: mergeIntoCache() unconditionally evicted every event in the
fetched date range, even for calendars whose sync failed. Those calendars'
events were removed and never restored, so the calendar appeared to vanish.
Fix:
- SyncError now decodes calendarId from the server's "calendar_id" field
(added server-side in e0ea16f but not yet read by iOS).
- mergeIntoCache() gains a keepKeysInRange parameter: events from failed
calendars are retained (not evicted) even within the fetch window.
- loadEvents() and prefetchBackground() compute the failed keys from the
syncErrors list and pass them to mergeIntoCache.
Result: when CalDAV / Google / HA sync fails for a specific calendar, the
user sees stale-but-correct events alongside the existing error banner,
instead of a completely empty calendar with no explanation.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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