Parse k=, h=, a= tags and derive RSA key bit-length from the public key
so consumers can detect weak configurations (SHA-1, short keys).
Scoring now penalises rsa-sha1 (cap 60), RSA <1024 bit (cap 25), and
RSA <2048 bit (cap 75); Ed25519 receives no penalty.
Fixes: #37
Remove authResults parameter from AnalyzeDNS, making it independent of
the authentication analysis step. Instead, parse DKIM-Signature headers
directly to extract domain and selector.
Bug: https://github.com/happyDomain/happydeliver/issues/11
