feat: firewalled peer connections via server-brokered connect

Implement the FIREWALLED connection mode: when a direct outbound connection to
a peer fails (peer unreachable), fall back to asking the server to broker it.

- Add MessageFactory::build_connect_to_peer (server code 18).
- A pre-established outbound failure now emits ClientOperation::PeerConnectFailed
  instead of a normal disconnect; the client registers a correlation token and
  sends ConnectToPeer so the peer connects back.
- The listener accepts the peer's PierceFirewall (peer code 0) connect-back,
  matches the pending token, registers it as the P control connection, and
  fires PeerConnected to flush the queued download.

Fix a latent aliasing bug this surfaced: a peer actor could emit two terminal
notifications (a connect-failure, then a disconnect when later replaced and
stopped), and the second removed the *newer* actor registered under the same
username, killing the brokered connection mid-transfer. An actor now reports a
terminal outcome at most once (disconnect_reported guard).

Verified end to end against soulfind: the mock advertises an unlistened port so
the direct connect is refused, the server brokers the connection, the mock
pierces back, and the file transfers — bytes match. A companion test
(soulfind_brokers_connect_to_peer_between_users) pins the server behaviour the
mode relies on.

soulseek-rs-lib/src/message/server/message_factory.rs

@@ -80,6 +80,22 @@ impl MessageFactory {
         Message::new().write_int32(23).write_int32(id).clone()
     }
 
+    /// Ask the server (code 18) to broker a connection to a firewalled peer:
+    /// the server tells that peer to connect back to us, quoting `token`.
+    #[must_use]
+    pub fn build_connect_to_peer(
+        token: u32,
+        username: &str,
+        connection_type: ConnectionType,
+    ) -> Message {
+        Message::new()
+            .write_int32(18)
+            .write_int32(token)
+            .write_string(username)
+            .write_string(&connection_type.to_string())
+            .clone()
+    }
+
     #[must_use]
     pub fn build_set_status_message(status_code: u32) -> Message {
         Message::new()