SyncManager

Uploads the queue of unsent packages (UnsentData).

Every pass runs on ONE worker thread, so passes never overlap: an item is read, posted and deleted by one pass before the next pass reads the queue (no double upload). Triggers (#312, ruling W10): SDK start (start), the network coming back (debounced), a newly queued package, and the in-session backoff retry. Triggers that arrive while a flush is already waiting to run are coalesced into it, so a burst of triggers costs at most one extra pass.