fix: stop the previous peer actor when re-registering a username

register_peer inserted the new handle and silently dropped the old one, leaving
the previous actor running as an orphan that pins a thread-pool worker forever;
its later shutdown could also remove the replacement connection from the
registry. The displaced actor is now told to stop.

soulseek-rs-lib/src/actor/peer_registry.rs

@@ -49,7 +49,17 @@ impl PeerRegistry {
             .peers
             .lock_safe()
             .map_err(|e| format!("peer registry lock poisoned: {}", e))?;
-        peers.insert(username.clone(), handle.clone());
+        // Stop any actor already registered under this username so it does not
+        // become an orphan pinning a pool worker forever (and so its later
+        // shutdown cannot remove this new connection from the registry).
+        if let Some(old_handle) = peers.insert(username.clone(), handle.clone())
+        {
+            let _ = old_handle.stop();
+            debug!(
+                "[peer_registry] Replaced existing peer actor for {}",
+                username
+            );
+        }
 
         Ok(handle)
     }