GetShareFileList (peer code 4) previously replied with a malformed message (a
server SharedFolders stats message over a peer connection). It now emits
ShareListRequested, and the client builds a real SharedFileListResponse
(code 5) from its shares grouped by directory. Adds a std-only code-5
encoder/decoder (build_shared_file_list / parse_shared_file_list) with a
roundtrip test, and Shares::directories() to group files. The decoder is the
foundation for the Browse feature.