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>

test/e2e/sink/Dockerfile

@@ -2,14 +2,17 @@
 # each transaction to its own file under /mail, so the test harness can read
 # the raw received message (headers + body, including the DKIM-Signature
 # Postfix/OpenDKIM added) straight off disk. smtp-sink ships in the postfix
-# package itself — no new dependency.
+# package itself. python3-dkim (dkimpy 1.1.8-2 in trixie, /usr/bin/dkimverify)
+# is the independent DKIM verifier the suite holds the panel's signer to
+# (docs/plans/panel-dkim-signing.md); it looks the keys up through the
+# stand's CoreDNS (compose.override.yml, sink's dns).
 FROM debian:trixie-slim
 
 ENV DEBIAN_FRONTEND=noninteractive
 RUN echo "postfix postfix/mailname string localhost" | debconf-set-selections \
  && echo "postfix postfix/main_mailer_type string Internet Site" | debconf-set-selections \
  && apt-get update \
- && apt-get install -y --no-install-recommends postfix \
+ && apt-get install -y --no-install-recommends postfix python3-dkim \
  && rm -rf /var/lib/apt/lists/*
 
 RUN mkdir -p /mail