logtail: count the message a hung sender check defers in cleanup (audit M13)

Seen on the e2e stand, the panel frozen with SIGSTOP once DATA was
answered: the client gets 451 4.7.1 after 30 s, and the warning that names
the socket is cleanup's, not smtpd's:

  postfix/cleanup[1339]: warning: milter unix:/run/selfpost/sender.sock:
    can't read SMFIC_HEADER reply packet header: Connection timed out
  postfix/cleanup[1339]: 7BCAF3C75DA0: milter-reject: END-OF-MESSAGE ...
    4.7.1 Service unavailable - try again later; ...

The counter read smtpd lines only, so Health's "messages deferred" missed
the one case that tells a hung panel from a stopped one. milterLineRe now
takes smtpd and cleanup; cleanup's "milter unix:<socket>:" warning counts a
message each time (one cleanup process serves message after message, and the
first warning ends the message); smtpd keeps once per connection. The
composed fixture is replaced by the seen lines, with two messages through one
cleanup process and OpenDKIM hung in cleanup (not counted).

e2e panel_hung_mid_message_is_counted: freezes the panel after DATA's 354,
wants a 4xx at the end of DATA, logs what mail.log says, and waits for
Health's today count to grow. kill through sh -c: the image has no kill(1).

CHANGELOG [Unreleased], audit plan line ticked, roadmap 13/61.
Evidence on the test server for this tree (with the next commit's docs):
go vet clean, go test ./... 23 packages ok, make e2e exit 0 -
renewed_certificate_picked_up 1.2 s, panel_hung_mid_message_is_counted
31.7 s.

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

CHANGELOG.md

@@ -37,6 +37,13 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
   key belong together, renews the copy and reloads Postfix; when nothing
   changed it no longer reloads (audit 2026-10-12, H2).
 
+- **Health counts mail deferred by a panel that hangs mid-message.** The
+  figure "messages deferred" on the Sender check card read only smtpd's
+  lines, but once a message's DATA has begun it is cleanup that talks to the
+  check, and its warning was never counted - the very case that tells a hung
+  panel from a stopped one. Seen on the e2e stand with the panel frozen, and
+  now counted, once per message (audit 2026-10-12, M13).
+
 ### Added
 
 - **The delivery log measures its own guesses** (migration `0007`). A table