check_upnp_available uses a blocking std UdpSocket and, on a network
with no UPnP gateway (the normal case right after a node moves), runs
out its full 3s read timeout. Inline on the runtime that blocked a
worker on every call, from four call sites. Same class as the OpenWrt
SSH stall (e282c059), smaller blast radius — move it to spawn_blocking.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
