Publish a Rust crate to crates.io: metadata, dry run, yanking

Documented steps

1. Fill in package metadata in Cargo.toml — crates.io requires a description and a license (or license-file); cargo warns at package time: 'manifest has no description, license, license-file, documentation, homepage or repository'. See https://doc.rust-lang.org/cargo/reference/manifest.html#package-metadata
2. Authenticate once with `cargo login`, pasting an API token generated at crates.io (Account Settings → API Tokens).
3. Validate everything without uploading: `cargo publish --dry-run` (packages the crate and builds it from the packaged sources).
4. Publish for real with `cargo publish`. Each version is immutable once uploaded — fixes require a new version number.
5. Un-publish new downloads of a bad version with `cargo yank <crate> --version <ver>`; reverse with `cargo yank --undo`. Yanking hides the version from new resolution but does not delete code or break existing Cargo.lock users.
6. Docs: https://doc.rust-lang.org/cargo/reference/publishing.html