Multi-file torrent support

[fraidev]

• Only handles single-file torrents (length field)
• Most torrents contain multiple files
• Missing files[] array handling

[fraidev]

1. Current Structure: The Info struct in file.rs already has support for multi-file torrents with the files: Option<Vec> field
2. Single-file assumption: The Torrent struct in torrent.rs only stores a single length field from torrent_meta.torrent_file.info.length.unwrap()
3. File writing: The CLI creates a single file and writes pieces sequentially to it
4. Piece calculation: utils::calculate_bounds_for_piece assumes a single contiguous file

[fraidev]

#28