core: bound rendezvous_register with an 8s timeout (was hanging iOS)

reqwest::Client::new() has no default timeout, so a slow/unreachable rendezvous
wedged IrohTransport.start() right after coming online — the transport never
reached "bootstrapping"/"joined topic". This is what hung the iOS app (both the
device and the simulator) while the macOS agent happened to be fast enough not to
notice. Build a client with an 8s timeout and add a "transport ready; registering"
log to pinpoint it. With the fix, the iPhone joins the gossip topic in seconds and
clipboard syncs phone⇄Mac over iroh (verified on device + simulator).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/src/iroh.rs

@@ -99,7 +99,12 @@ async fn rendezvous_register(server: &str, room: &str, my_id: &str) -> Vec<Endpo
     }
     let url = format!("{}/rendezvous/register", server.trim_end_matches('/'));
     let body = serde_json::json!({ "room": room, "id": my_id });
-    let mut req = reqwest::Client::new().post(&url).json(&body);
+    // Bounded — a stuck rendezvous must never wedge the transport.
+    let client = reqwest::Client::builder()
+        .timeout(std::time::Duration::from_secs(8))
+        .build()
+        .unwrap_or_default();
+    let mut req = client.post(&url).json(&body);
     // Optional bearer token, matching the rendezvous's TETHER_RENDEZVOUS_TOKEN.
     if let Ok(token) = std::env::var("TETHER_RENDEZVOUS_TOKEN") {
         if !token.is_empty() {
@@ -189,6 +194,7 @@ impl Transport for IrohTransport {
             });
 
             // Bootstrap: env override (tests) + the rendezvous (zero-config).
+            eprintln!("tethercore[iroh]: transport ready; registering with rendezvous {}", this.cfg.server);
             let mut boot = env_bootstrap_ids();
             boot.extend(rendezvous_register(&this.cfg.server, &this.cfg.room, &my_id).await);
             eprintln!("tethercore[iroh]: bootstrapping with {} peer(s)", boot.len());