Embed secure hashes in all torrents

Sha1 and MD5 are broken, so it might be prudent to embed secure hashes into torrents. These might not be immediately useful, but could allow someone with the torrent to verify that data is retrieved successfully.

Start by examining the torrent creation and verification paths in this Rust CLI and how torrent metadata is represented. Determine the supported secure hash approach and the expected verification behavior. Done means torrents embed the agreed hashes and verification can use them, with tests covering the new metadata and validation.

Written by the indexing model from the issue text.