logtail, health, panel: cite the source of what they rely on (assumptions part A)

Comments only, no behaviour changed. Each place the log tailer, Health and
cmd/panel rely on another program now says where that is written, for the
version the image runs: Postfix 3.7.11 (log_adhoc(), msg_logger_print(),
smtpd_whatsup(), cleanup_milter.c, qmgr_active.c, postlogd.c, showq.c,
pipe.c), logrotate 3.21.0, supervisor 4.2.5, proc(5). Measurements stay
measurements and say where and when; test fixtures say whether they were
seen or composed.

Where the source contradicts the code the comment does not bless it but
names the register entry (A-8 expiry, A-12 the ISO stamp no Postfix writes,
A-15 the last recipient, A-21/A-23 sasl_username, A-33 postlogd's old
inode, A-38 a socket file is not a listener, A-65 the certificate served);
those are fixed in their own commits.

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

internal/logtail/sendercheck_test.go

@@ -37,7 +37,9 @@ func (d *deferredStore) AddSenderCheckDeferred(at time.Time, n int64, place stri
 // panel frozen in the middle of a message (cleanup 1339, 2026-10-14); the
 // stamps are put in ISO form. Around them are the lines of two other things
 // that use the same words - a level-2 refusal of the journal milter, OpenDKIM
-// not answering.
+// not answering. Those - smtpd 1149, 1190 and 1201, the second client of
+// 1183, smtpd 1300 and the OpenDKIM warning of cleanup 1339 - are composed,
+// after src/smtpd/smtpd.c and src/milter/milter8.c (Postfix 3.7.11).
 func TestDeferredCounter(t *testing.T) {
 	st := &deferredStore{}
 	c := newDeferredCounter(&SenderCheck{Store: st, Socket: "/run/selfpost/sender.sock"})