panel-dkim-signing stage 2: OpenDKIM removed

The panel's signer is the only one. Gone: the opendkim package and its
configuration, the syslog-relay and opendkim programs, /dev/log and
/run/opendkim, the reload by USR1 through supervisord
(internal/supervisor/supervisor.go), internal/syslogrelay, the port of
OpenDKIM's From: parser (internal/milter/mailparse.go and its probe,
harness and test/upstream scripts), OPENDKIM_SOCKET and OPENDKIM_DIR.

Keys: internal/domain/keys.go (Keys, NewKeys, DKIM_DIR=/data/dkim) keeps
the keys and writes a selector table, /data/dkim/selectors ("<name>
<selector>"), which the signer reads in place of the KeyTable - it reads
no database, as the sender check reads none. The table is written before
the signer serves (EnsureServerSigned moved there from the notifier).
Keys and table 0600, directories 0700, the panel's. The entrypoint copies
/data/opendkim/keys once, as the panel user, into /data/dkim.new renamed
when whole, and stops the start if the copy fails; /data/opendkim is left
for a rollback to 2.7.x.

Sender check: one RFC 5322 address as a mail program reads it, held
against the bindings; the 256-byte refusal and the texts naming OpenDKIM
go. Allowed() folded the domain through strings.ToLower, which makes the
Kelvin sign a "k" - the comparison with OpenDKIM's reading had hidden it;
it folds ASCII only now.

Health: two dkim-signer rows (applications, notifications) in Milter
sockets, opendkim out of the liveness set; the card no longer takes any
row with a note for the spam filter. Texts as in the design commit.

KILL stays in deploy/docker-compose.yml: measured on the test server,
root without it cannot signal a process of another uid - supervisord
stopping the panel, Postfix's master its children. The plan's "only use
was the reload" was wrong; the comment now gives the real uses.

e2e: one signature, the panel's, verified by dkimpy; the /data/dkim modes;
the OpenDKIM log and process checks, testNeverSignedForAnother and the
cited opendkim, libopendkim11 and libmilter1.0.1 go.

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

deploy/docker-compose.yml

@@ -103,7 +103,7 @@ services:
     # `user: panel` or a fully read-only rootfs without breaking that startup
     # self-healing. What IS applied: no privilege escalation past what the
     # image already grants, and every Linux capability dropped except the
-    # small set the root startup phase and Postfix/OpenDKIM genuinely need:
+    # small set the root startup phase and Postfix genuinely need:
     #   - NET_BIND_SERVICE — bind 465/587 and inbound 25 below 1024;
     #   - CHOWN            — entrypoint re-owns /data (bind mount) to `panel`;
     #   - FOWNER           — entrypoint then chmods those now panel-owned /data
@@ -113,12 +113,18 @@ services:
     #   - SETUID/SETGID    — supervisord drops the panel to the unprivileged
     #                        `panel` user; Postfix switches to its own users;
     #   - DAC_OVERRIDE     — cross-user file access within the `selfpost` group.
-    #   - KILL             — supervisord (root) sends SIGUSR1 to opendkim (a
-    #                        different uid) on domain add/remove to reload its
-    #                        KeyTable/SigningTable; without it os.kill() fails
-    #                        with EPERM even though the caller is root, because
-    #                        the kernel's signal permission check for
-    #                        cross-uid kill() still consults CAP_KILL.
+    #   - KILL             — root signals processes of other uids: supervisord
+    #                        stops and restarts the panel (supervisor 4.2.5,
+    #                        supervisor/process.py:467, options.kill()), and
+    #                        Postfix's master ends its children, which run as
+    #                        `postfix` (Postfix 3.10.13, src/master/master_sig.c:196,
+    #                        master_sigdeath(); master_spawn.c:365,
+    #                        master_delete_children()). Without it kill() fails
+    #                        with EPERM although the caller is root (kill(2),
+    #                        capabilities(7); measured on the test server
+    #                        2026-10-16: root with SETUID/SETGID and no KILL
+    #                        cannot kill a process of uid 65534). Until 2.8.0
+    #                        it also carried OpenDKIM's USR1 reload.
     # FOWNER/FSETID are required by build/entrypoint.sh's permission
     # self-healing; without them chmod fails with EPERM and the container
     # crash-loops on start.