Verify that the From and Return-Path domains can actually receive replies
and bounces, mirroring Fastmail's authentication_milter ReturnOK handler.
Each domain is checked for MX records, falling back to A/AAAA (implicit MX)
and then to the organizational domain, yielding a pass/warn/fail status.
Adds return_ok to DNSResults, a 10-point DNS sub-score penalty per domain
that is wholly unreachable, and a new "Return Address Reachability" card.
Compare the HELO/EHLO hostname announced by the sending server (first
Received hop) against the sender IP's PTR records, surfacing the same
signal as x-ptr/policy.ptr in Authentication-Results. Adds helo_hostname
and helo_ptr_match to DNSResults, applies a 15-point PTR sub-score
penalty on mismatch, and displays the result in a new HELO/PTR
Consistency card.
DMARC lookup now follows the full RFC 7489 §6.6.3 fallback chain: exact
From domain → organizational domain (eTLD+1 via PSL) → public suffix
domain (RFC 9091, only when psd=y is present). DNS errors abort
immediately without triggering fallback; NXDOMAIN and missing v=DMARC1
records do trigger it. The found domain is exposed in the new
DMARCRecord.domain field for reporting purposes.
Also promote getOrganizationalDomain to a package-level function so both
HeaderAnalyzer and DNSAnalyzer can share it, and fix pre-existing
rbl_test.go compilation errors and stale score expectations.
Closes: #98
