v0.7.3

chore(release): bump to 0.7.3 for v0.7.3 tag

Bump Cargo.toml to 0.7.3 so the v0.7.3 tag carries a binary that
reports 0.7.3 (the original v0.7.0 packaging bug was exactly this
pattern — tag carrying a binary that reported an older version).

Bump the action's lain-version default to v0.7.3 in the same
commit. The version-drift job in ci.yml would go red on the
next push after tagging otherwise; bumping here keeps the
post-tag drift check green.