feat: direct peer connections — download() connects to unconnected peers

Implement the DIRECT connection-initiation mode so a download can target a
peer the client is not already connected to:

- Fix the outbound peer handshake. on_connection_established sent
  build_watch_user() (a server WatchUser message, wrongly framed with
  get_data()) for outbound connections; it now sends a proper length-prefixed
  PeerInit (peer code 1) with our own username. own_username is plumbed in
  via PeerRegistry -> PeerActor; inbound peers (outbound=false) are unchanged.
- download() now auto-connects: if there is no control connection to the peer
  it sends GetPeerAddress and defers the QueueUpload until the outbound stream
  handshakes, signalled by a new ClientOperation::PeerConnected. A never-
  connected client still resolves the download to Failed (unit test).
- Add Client::connect_peer(username) to open/pre-warm a control connection.

Verified end to end against soulfind: a_file_downloads_from_a_peer_via_direct_connection
logs a mock peer into the server, advertises its port via SetWaitPort, and the
client resolves the address, dials it, and downloads the file — bytes match.
The F-serve leg is shared with the passive-download mock.

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

@@ -142,7 +142,6 @@ impl MessageFactory {
             .clone()
     }
 
-    #[allow(dead_code)]
     #[must_use]
     pub fn build_peer_init_message(
         own_username: &str,
@@ -184,6 +183,22 @@ fn test_build_login_message() {
     assert_eq!(expect, message.get_data());
 }
 
+#[test]
+fn test_build_peer_init_message() {
+    use crate::peer::ConnectionType;
+    let message =
+        MessageFactory::build_peer_init_message("bob", ConnectionType::P, 7);
+    // [1][len=3]"bob"[len=1]"P"[token=7] — no length prefix in get_data()
+    let expect: Vec<u8> = [
+        1, // PeerInit code (int8)
+        3, 0, 0, 0, 98, 111, 98, // username "bob"
+        1, 0, 0, 0, 80, // connection type "P"
+        7, 0, 0, 0, // token
+    ]
+    .to_vec();
+    assert_eq!(expect, message.get_data());
+}
+
 #[test]
 fn test_build_message_user() {
     let message = MessageFactory::build_message_user("bob", "hi");