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/client/operations.rs

@@ -492,6 +492,36 @@ impl Client {
                                     e
                                 ),
                             },
+                            ClientOperation::UserStatusReceived {
+                                username,
+                                status,
+                                privileged,
+                            } => match client_context.write_safe() {
+                                Ok(mut ctx) => ctx.apply_user_status(
+                                    username, status, privileged,
+                                ),
+                                Err(e) => error!(
+                                    "[client] UserStatusReceived write: {}",
+                                    e
+                                ),
+                            },
+                            ClientOperation::UserStatsReceived {
+                                username,
+                                average_speed,
+                                shared_files,
+                                shared_folders,
+                            } => match client_context.write_safe() {
+                                Ok(mut ctx) => ctx.apply_user_stats(
+                                    username,
+                                    average_speed,
+                                    shared_files,
+                                    shared_folders,
+                                ),
+                                Err(e) => error!(
+                                    "[client] UserStatsReceived write: {}",
+                                    e
+                                ),
+                            },
                             ClientOperation::RoomEvent(event) => {
                                 match client_context.write_safe() {
                                     Ok(mut ctx) => ctx.apply_room_event(event),