panel-dkim-signing stage 1: the panel's DKIM signer, beside OpenDKIM

internal/milter/sign.go: a milter on two sockets (cmd/panel/signer.go):
sign.sock for 465/587, the key of the domain the SASL login belongs to
(Bindings.DomainOf, the sender_login_maps Postfix reads), and
sign-local.sock for the panel's own sendmail(1) mail, the server's name.
go-msgauth/dkim v0.7.0, relaxed/simple, OpenDKIM's dkim_should_signhdrs[]
(libopendkim/dkim.c:223-246) in message order plus From once more; keys
and selectors from the KeyTable the panel writes, a key path outside the
key directory refused; strict - whatever cannot be signed is deferred.

From the sources: Postfix 3.10.13 milter8.c:2329-2400 hands a folded
header \n-separated, so the signer joins its lines with CRLF; inserts are
1-based in cleanup_ins_header() (cleanup_milter.c:1193), so index 0 is the
top, where OpenDKIM puts its own (opendkim.c:3673). go-milter v0.4.1 calls
no Abort when a connection ends, so a watchdog closes go-msgauth's
streaming signer after an hour and a late end of message is deferred.

Postfix: smtpd_milters = sender check, signer, OpenDKIM, journal;
non_smtpd_milters = the signer's local socket, OpenDKIM - both sign every
message until stage 2. disable_mime_output_conversion=yes (default off,
mail_params.h:3088): an 8-bit message is no longer re-encoded after it is
signed.

Tests: unit tests sign and verify folded, tab-led and space-free headers,
an added From:, a changed body, two messages on a connection, odd bodies,
and five cases deferred. e2e: the sink image has python3-dkim (dkimpy
1.1.8-2) and the stand's DNS; on an application's message and on a
notification the two signatures must agree on h=, bh=, c=, s=, a= and each
must pass dkimverify. On the server: make e2e exit 0 (714 s), go vet + go
test 27 packages ok.

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

CHANGELOG.md

@@ -5,6 +5,24 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ## [Unreleased]
 
+### Added
+
+- **The panel signs outgoing mail with DKIM itself** (panel-dkim-signing,
+  stage 1 of [the plan](docs/plans/panel-dkim-signing.md)): a milter of the
+  panel's on two sockets, one for applications' mail - signed with the key of
+  the domain the login belongs to - and one for the panel's own, signed with
+  the server's name. `relaxed/simple`, RFC 6376 § 5.4.1's headers and `From`
+  once more, as OpenDKIM signs; strict. Until OpenDKIM leaves the image both
+  sign every message, and the e2e suite holds the two to the same `h=` and
+  `bh=` and has dkimpy - an independent implementation - verify each.
+
+### Fixed
+
+- **An 8-bit message could leave with a broken signature.** Postfix rewrote
+  it as quoted-printable for a server that does not announce 8BITMIME, after
+  it was signed; `disable_mime_output_conversion=yes` now delivers it as
+  signed. OpenDKIM's signatures were exposed to this too.
+
 ## [2.7.0] - 2026-10-15
 
 The image runs Debian trixie and Postfix 3.10.13, with upstream's 2026