feat(lib): join the distributed search network as a leaf

The server hands every client a list of possible parents and expects it to
hang from one so searches also travel peer to peer. Until now we told the
server we were a lone root and left it at that, which the network tolerates
but does not favour.

A leaf now dials each PossibleParents candidate over a D connection, adopts
the first that passes a search down after stating its branch, announces the
new branch root and level to the server, and answers those searches the same
way as ones from the server. Losing the parent or a ResetDistributed puts it
back to a parentless stance. The leaf takes no children.

The Leaf state machine is pure so its rules are unit-tested without sockets;
the e2e test drives a real client from a mock parent, since soulfind never
hands out parents.

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

@@ -4,6 +4,7 @@ use crate::dispatcher::MessageDispatcher;
 use crate::message::server::AdminMessageHandler;
 use crate::message::server::CheckPrivilegesHandler;
 use crate::message::server::ConnectToPeerHandler;
+use crate::message::server::EmbeddedMessageHandler;
 use crate::message::server::ExcludedSearchPhrasesHandler;
 use crate::message::server::FileSearchHandler;
 use crate::message::server::GetPeerAddressHandler;
@@ -14,8 +15,10 @@ use crate::message::server::MessageFactory;
 use crate::message::server::MessageUser;
 use crate::message::server::ParentMinSpeedHandler;
 use crate::message::server::ParentSpeedRatioHandler;
+use crate::message::server::PossibleParentsHandler;
 use crate::message::server::PrivilegedUsersHandler;
 use crate::message::server::ReloggedHandler;
+use crate::message::server::ResetDistributedHandler;
 use crate::message::server::SayChatroomHandler;
 use crate::message::server::UserJoinedRoomHandler;
 use crate::message::server::UserLeftRoomHandler;
@@ -156,6 +159,11 @@ pub enum ServerMessage {
         room: String,
         username: String,
     },
+    /// Peers the server suggests as distributed-network parents: username,
+    /// host, port.
+    PossibleParents(Vec<(String, String, u16)>),
+    /// The server asks us to drop our parent and start over.
+    ResetDistributed,
 }
 
 pub struct ServerActor {
@@ -205,14 +213,9 @@ fn post_login_messages(
 }
 
 /// What a leaf tells the server while it has no parent: it is its own branch
-/// root at level 0 and takes no children. Nicotine+ sends the same four.
+/// root at level 0.
 fn parentless_stance(own_username: &str) -> Vec<Message> {
-    vec![
-        MessageFactory::build_have_no_parent(true),
-        MessageFactory::build_branch_root(own_username),
-        MessageFactory::build_branch_level(0),
-        MessageFactory::build_accept_children(false),
-    ]
+    crate::message::distributed::stance(own_username, 0, false)
 }
 
 impl ServerActor {
@@ -306,6 +309,9 @@ impl ServerActor {
         handlers.register_handler(LoginHandler);
         handlers.register_handler(ReloggedHandler);
         handlers.register_handler(AdminMessageHandler);
+        handlers.register_handler(PossibleParentsHandler);
+        handlers.register_handler(ResetDistributedHandler);
+        handlers.register_handler(EmbeddedMessageHandler);
         handlers.register_handler(RoomListHandler);
         handlers.register_handler(GetUserStatusHandler);
         handlers.register_handler(WatchUserHandler);
@@ -517,6 +523,14 @@ impl ServerActor {
     /// length; there is no behaviour here beyond dispatch.
     fn handle_standing_message(&mut self, message: ServerMessage) {
         match message {
+            ServerMessage::PossibleParents(candidates) => {
+                self.forward_to_client(ClientOperation::PossibleParents(
+                    candidates,
+                ));
+            }
+            ServerMessage::ResetDistributed => {
+                self.forward_to_client(ClientOperation::ResetDistributed);
+            }
             ServerMessage::WishlistSearch { token, query } => {
                 self.queue_message(MessageFactory::build_wishlist_search(
                     token, &query,