The checksum model only carried md5/sha1/sha256 while deb-buildinfo(5)
defines Checksums-Sha512, and there was no way to parse a Checksums-*
field body back into entries. Add ChecksumKind::Sha512 (computed
alongside the others), a field parser validating the
'<hex> <size> <name>' grammar, and an only-if-populated
Checksums-Sha512 emission in .buildinfo — deliberately dormant in the
dpkg-parity flows, which never emit it, and .dsc/.changes untouched.