feat(lib): send and receive private messages

Wire private messaging through the client:

- MessageFactory::build_message_user (code 22) and build_message_acked
  (code 23);
- the incoming MessageUser handler now acknowledges freshly delivered
  messages and forwards them to the client instead of only logging them;
- ClientContext buffers received UserMessages;
- Client::send_private_message and Client::take_private_messages expose the
  feature, and UserMessage (with accessors) is re-exported from the crate.

Verified end to end against soulfind: a_private_message_is_delivered_between_users
logs in two clients and asserts one receives the other's message.

soulseek-rs-lib/tests/e2e.rs

@@ -251,6 +251,42 @@ fn a_search_is_forwarded_to_a_connected_peer() {
     assert!(receiver.get_all_searches().contains_key(probe));
 }
 
+#[test]
+fn a_private_message_is_delivered_between_users() {
+    let server = server_or_skip!();
+
+    // Two logged-in users, one messages the other through the server.
+    let mut alice =
+        Client::with_settings(server.settings("e2e_alice_pm", "pw"));
+    let mut bob = Client::with_settings(server.settings("e2e_bob_pm", "pw"));
+    alice.connect().expect("alice connect");
+    bob.connect().expect("bob connect");
+    assert!(alice.login().expect("alice login"));
+    assert!(bob.login().expect("bob login"));
+
+    let body = "hello bob, this is alice";
+    alice
+        .send_private_message("e2e_bob_pm", body)
+        .expect("send private message");
+
+    // Delivery is asynchronous; poll Bob's inbox until the message arrives.
+    let deadline = Instant::now() + Duration::from_secs(5);
+    let mut received = Vec::new();
+    while Instant::now() < deadline {
+        received.extend(bob.take_private_messages());
+        if !received.is_empty() {
+            break;
+        }
+        std::thread::sleep(Duration::from_millis(100));
+    }
+
+    let message = received
+        .iter()
+        .find(|m| m.message() == body)
+        .expect("bob should receive alice's message");
+    assert_eq!(message.username(), "e2e_alice_pm");
+}
+
 #[test]
 fn login_succeeds_with_listener_enabled() {
     let server = server_or_skip!();