Rebased on stage 6b. The hook of the signature of alg 1, which 6b named
AuthorKey, is now AuthorSigner, and AuthorKey of authorkey.dart implements
it. The tests of the writer sign the capsules of alg 1 with the AuthorKey
of Go's seed instead of replaying the recorded signature, and write the
bytes of Go.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>