feat(protocol): interests, tickers, global room and cant-connect

Adds the server messages the client had no way to speak: user and room
search (42/120), room tickers (113-116), the global room feed (150-152),
interests and the recommendation queries built on them (51/52/54/56/57/
110/111/112/117/118), a keepalive ping (32), gifted privileges (123), a
password change (142) and multi-user private messages (149).

CantConnectToPeer (1001) closes a real gap in both directions: a dial the
server asked us to make now answers the waiting peer instead of falling
back to brokering the same unreachable link the other way, and an incoming
one ends the wait on downloads queued for that peer rather than letting
them sit until a timeout.

soulseek-rs-lib/src/client/search.rs

@@ -81,6 +81,55 @@ impl Client {
         Ok(())
     }
 
+    /// Search one user's shares (server code 42) and return at once.
+    ///
+    /// Results arrive from that user alone and accumulate under `query`, read
+    /// back with [`Client::get_search_results`] — the same place a plain
+    /// search's results land, since the replies are ordinary
+    /// `FileSearchResponse` frames.
+    ///
+    /// # Errors
+    /// [`SoulseekRs::NotConnected`] when there is no server connection.
+    pub fn search_user(&self, username: &str, query: &str) -> Result<()> {
+        let token = self.register_search(query)?;
+        self.send_server_message(
+            crate::message::server::MessageFactory::build_user_search(
+                username, token, query,
+            ),
+        )
+    }
+
+    /// Search the shares of everyone in `room` (server code 120), returning at
+    /// once. Results accumulate under `query` exactly as above.
+    ///
+    /// # Errors
+    /// [`SoulseekRs::NotConnected`] when there is no server connection.
+    pub fn search_room(&self, room: &str, query: &str) -> Result<()> {
+        let token = self.register_search(query)?;
+        self.send_server_message(
+            crate::message::server::MessageFactory::build_room_search(
+                room, token, query,
+            ),
+        )
+    }
+
+    /// Take a token for `query` and register it, so replies quoting that token
+    /// are filed under `query` when they arrive.
+    fn register_search(&self, query: &str) -> Result<u32> {
+        if self.server_handle.is_none() {
+            return Err(SoulseekRs::NotConnected);
+        }
+        let token = next_search_token();
+        self.context.write_safe()?.searches.insert(
+            query.to_string(),
+            Search {
+                token,
+                results: Vec::new(),
+            },
+        );
+        Ok(token)
+    }
+
     /// Let responses accumulate for `timeout`, or until cancelled.
     ///
     /// Peers answer a search over their own connections whenever they get round