fix(lib): advertise real slot, speed and queue figures in search replies

Every reply to a peer's search claimed a free upload slot, no speed and
an empty queue, whatever the sharer was doing, and searchers rank
sources on exactly those fields. A reply now says whether a slot is
free, the bytes per second of the last completed upload, and how many
uploads are waiting.

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

@@ -123,7 +123,7 @@ impl ClientContext {
         mut next_token: impl FnMut() -> u32,
     ) -> (Option<PeerRegistry>, Vec<UploadOffer>) {
         let mut offers = Vec::new();
-        while self.uploads_in_flight() < self.upload_slots {
+        while self.has_free_upload_slot() {
             let Some(index) = next_to_serve(&self.upload_queue) else {
                 break;
             };
@@ -190,6 +190,10 @@ impl ClientContext {
                 .count()
     }
 
+    pub(super) fn has_free_upload_slot(&self) -> bool {
+        self.uploads_in_flight() < self.upload_slots
+    }
+
     /// The peers still waiting, in the order they will be served, each carrying
     /// its own 1-based place.
     pub(super) fn queued_uploads(&self) -> Vec<UploadInfo> {