feat(cli): expose sharing, peer lookup and settings as one-shot commands

An agent driving the binary could search, fetch and chat, but it could not be
a peer: nothing kept a session online to serve files, nothing reported an
upload, shares could only be set at startup, and there was no way to ask
whether a peer was worth queueing to or to check credentials without doing
something else.

New commands:

  serve [--follow]                stay online sharing; one record per upload
  whoami                          credentials, connection and what we offer
  user <NAME>                     a peer's presence and share counts
  room users <ROOM>               the member roster
  shares list|add|remove|status|reindex
  config path|list|get|set

`config` and `shares list|add|remove` need no account, so they run before the
credential check — asking a script to log in for a `config get` would be a
poor trade. `serve` is the only place uploads are visible: they live in the
process serving them, so a one-shot `uploads list` could print nothing but an
empty list.

Library additions, all request/poll pairs matching the existing conventions:

- Client::room_members, kept current from the join and leave events
- Client::request_user_info / user_info, backed by new handlers for
  GetUserStatus (7) and GetUserStats (36) — codes the dispatcher previously
  logged as unhandled. The wire format is taken from soulfind's own source
  and pinned by an e2e test that asserts a peer's real share count.
- UserInfo carries presence and stats as Options, so a reply that never
  arrived cannot be read as "offline" or "shares nothing"
- request_user_info clears the previous answer, so the documented poll loop
  observes this request rather than the last one
- ServerMessage and ClientOperation are now #[non_exhaustive]: every protocol
  message the client learns adds a variant, and that should not break callers
- the crate root and prelude re-export every type a Client method returns

Also pins the output contract with tests: under --json, stdout carries only
newline-delimited JSON across fifteen command shapes while progress still
goes to stderr, and a failing command writes nothing to stdout at all.

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

@@ -122,6 +122,21 @@ impl MessageFactory {
             .clone()
     }
 
+    /// Ask the server (code 7) for a user's online status.
+    #[must_use]
+    pub fn build_get_user_status(username: &str) -> Message {
+        Message::new().write_int32(7).write_string(username).clone()
+    }
+
+    /// Ask the server (code 36) for a user's share statistics.
+    #[must_use]
+    pub fn build_get_user_stats(username: &str) -> Message {
+        Message::new()
+            .write_int32(36)
+            .write_string(username)
+            .clone()
+    }
+
     /// Ask the server (code 64) for the list of public chat rooms.
     #[must_use]
     pub fn build_room_list_request() -> Message {
@@ -228,6 +243,20 @@ impl MessageFactory {
     }
 }
 
+#[test]
+fn test_build_get_user_status() {
+    let message = MessageFactory::build_get_user_status("bob");
+    let expect: Vec<u8> = [7, 0, 0, 0, 3, 0, 0, 0, b'b', b'o', b'b'].to_vec();
+    assert_eq!(expect, message.get_data());
+}
+
+#[test]
+fn test_build_get_user_stats() {
+    let message = MessageFactory::build_get_user_stats("bob");
+    let expect: Vec<u8> = [36, 0, 0, 0, 3, 0, 0, 0, b'b', b'o', b'b'].to_vec();
+    assert_eq!(expect, message.get_data());
+}
+
 #[test]
 fn test_build_watch_user() {
     let token: u32 = 223;