mailrs-arc 1.2.0

RFC 8617 Authenticated Received Chain (ARC) — header parsing, chain extraction, and signature verification. ARC extends DKIM/SPF/DMARC across forwarders by chaining per-hop authentication results inside cryptographically-sealed header sets. Pairs with mailrs-dkim for canonicalization + RSA verify, mailrs-spf / mailrs-dmarc for the auth-results that ARC carries forward.