fix(network): SSDP discovery no longer parks a tokio worker for 3s

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>

core/archipelago/src/network/router.rs

@@ -97,7 +97,15 @@ async fn get_wan_ip() -> Option<String> {
 }
 
 /// Check if UPnP is available by attempting SSDP discovery.
+///
+/// The socket is a blocking `std::net::UdpSocket`, and on a network with no
+/// UPnP gateway — the normal case right after a node moves — the recv runs
+/// out its full read timeout. Inline on the runtime that parked a tokio
+/// worker for those 3s on every call, the same failure shape (smaller blast
+/// radius) as the OpenWrt SSH connect that stalled the API on framework-pt.
+/// Keep it on the blocking pool.
 async fn check_upnp_available() -> bool {
+    tokio::task::spawn_blocking(|| {
         use std::net::UdpSocket;
 
         let ssdp_request = "M-SEARCH * HTTP/1.1\r\n\
@@ -133,6 +141,9 @@ async fn check_upnp_available() -> bool {
             }
             Err(_) => false,
         }
+    })
+    .await
+    .unwrap_or(false)
 }
 
 /// Add a port forward (stored locally; actual UPnP mapping done on request).