Add the zip format of a package (a digest manifest plus the files of the
OCI package for one platform) and an addonpkg zip subcommand that writes
it from a verified layout, with a sha256 file. The zip comes from the
same layout the registry package comes from, so there is one build path.
Version comparison moves to contract.CompareVersions so the installer
can use it for upgrades and downgrades.
Assisted-by: Claude:claude-sonnet-5-5 go gofmt