Function start_sync 

pub fn start_sync(
    shared: &Arc<SharedRootHandle>,
    sync_id: u16,
    opts: SyncOptions,
    outbox: Outbox,
) -> SyncHandle

Spawn a sync engine subscribed to shared, streaming to outbox. The engine’s initial RESET … SYNC series is cut from the root’s current snapshot — later syncs of an already-watched root never rescan.