Publish ethlambda crates to crates.io once leanvm is no longer a git dependency #697

The release pipeline (#674) publishes to GitHub Releases and GHCR only. crates.io is not covered: it rejects any crate with a git dependency, and the workspace pulls leanvm (and the four libssz crates) from git.

Blocked on

• leanvm being published to crates.io (or vendored as a path dependency we publish ourselves).
• libssz returning to a crates.io release, which CLAUDE.md already tracks (feat: add DerefMut to SszList and SszVector types libssz#33, and feat: bump test fixtures to latest version #37 for the gloas stack).

When unblocked

• Decide which crates to publish (the binary, or also the library crates other clients might reuse, e.g. ethlambda-ssz-tree).
• Add a publish step to release_promote.yaml, so only promoted releases reach crates.io.
• [workspace.package].version is already checked against the tag, so versions stay consistent.