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/architecture.md

@@ -294,8 +294,10 @@ then OpenDKIM did, in a program of its own
 - **What is signed** — `relaxed/simple`, `rsa-sha256`, with
   `github.com/emersion/go-msgauth` v0.7.0 (`dkim`). `h=`: every header of
   the message whose name is in RFC 6376 § 5.4.1's list as OpenDKIM's
-  `dkim_should_signhdrs[]` held it, and `Sender` and `List-Unsubscribe-Post`
-  besides (`signHeaders`), each occurrence, in message order; then, once
+  `dkim_should_signhdrs[]` held it, and `Sender`, `List-Unsubscribe-Post`,
+  `Content-Type`, `MIME-Version` and `Message-ID` besides (`signHeaders`;
+  `Message-ID` is not over-signed: cleanup adds one a message lacks after the
+  milters), each occurrence, in message order; then, once
   more, each of `Subject`, `Date`, `To`, `Cc`, `Reply-To` and `Sender` the
   message has (`overSign` - only those present: Postfix's cleanup adds a
   missing `Date:` after the milters, out of their sight, and a name in `h=`