AddNode registers the connection before sending the initial map, so a
batched delta could land first and become the stream's first frame;
Tailscale clients then kill the poll with "initial MapResponse lacked
Node" and a retry loop under steady change traffic leaves the netmap
empty. Skip connections awaiting their initial map and retry the
change next tick — the in-flight map may predate it, so it cannot be
dropped. Retries are prepended to keep patch order.
Kristoffer Dalby