signer: MIME headers and Message-ID signed; a long header line logged (audit M6, M8)

Security audit 2026-10-17, M6 and M8 (nits), stage 6.

M6: Content-Type, MIME-Version and Message-ID were not in h= (as in
OpenDKIM's default list): a relay could change a MIME boundary or the type
and the signature still verified. They are signed when present; Message-ID
is not over-signed, since cleanup adds one a message lacks after the
milters and an absent name in h= would refuse it.

M8: a header line over smtp_line_length_limit (998) is broken by Postfix's
smtp client after signing and the signature fails silently; only body
lines were logged. longHeaderLine names the header, once per message, in
the same words.

Tests: TestSignLinesCoverTheMIMEHeadersAndLongHeaderLinesAreNamed; the h=
expectations gain Message-ID; e2e dkim_shapes expects content-type: in h=
(and the receiver's verification over it). Docs: security.md,
architecture.md, guide; CHANGELOG.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/security.md

@@ -324,8 +324,10 @@ operator can act on it.
   server's key may sign it. What cannot be signed (a login with no domain, a
   name with no key, a read error) is deferred, never sent unsigned. Port 25
   has no signer. `relaxed/simple`, rsa-sha256; `h=` names every header of
-  RFC 6376 § 5.4.1's list the message has, and `Sender` and
-  `List-Unsubscribe-Post`, then once more each of the shown ones present
+  RFC 6376 § 5.4.1's list the message has, and `Sender`,
+  `List-Unsubscribe-Post`, `Content-Type`, `MIME-Version` and `Message-ID`
+  (the last three since `2.14.4`, audit M6: unsigned, a relay could change a
+  MIME boundary and the signature still verified), then once more each of the shown ones present
   (Subject, Date, To, Cc, Reply-To, Sender) and `From` last, so one of those
   added after signing breaks the signature. Until `2.8.0` OpenDKIM signed - a
   2018 beta with three 2026 CVEs open and nobody upstream to fix them