feat(shares): encode FileSearchResponse (peer code 9)

Add build_file_search_response + a borrowed FileEntry view: the zlib-compressed
reply advertising our matching shared files to a searcher. The payload is the
exact inverse of SearchResult::new_from_message (username, token, per-file
name/size/ext/attributes, slots, speed), compressed with the stored-block
encoder. Round-trip tested through the production decoder.

soulseek-rs-lib/src/message/peer/mod.rs

@@ -7,7 +7,9 @@ mod transfer_response;
 mod upload_failed;
 
 // Re-export handlers
-pub use file_search_response::FileSearchResponse;
+pub use file_search_response::{
+    FileEntry, FileSearchResponse, build_file_search_response,
+};
 pub use get_share_file_list::GetShareFileList;
 pub use peer_init::PeerInit;
 pub use place_in_queue_response::PlaceInQueueResponse;