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.
Add a std-only shares module: Shares::scan walks the shared directory (skipping
symlinks and unreadable entries) into a read-only snapshot keyed by peer-facing
virtual path (root name + backslash-separated relative path). Exposes
search (case-insensitive multi-term AND), get (O(1) by virtual path), and
file/folder counts for the login handshake. Unit-tested against a temp tree.
michel