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/refusals_test.go

@@ -29,6 +29,11 @@ func (r *refusalProbe) AttachRefusalLine(clientIP, from string, _ time.Time, lin
 	return true, nil
 }
 
+// The mail.log lines in this file are composed by hand, with the e2e stand's
+// names and addresses (smtpd.c check_milter_reply(), mail_open_stream();
+// cleanup_milter.c cleanup_milter_apply(), Postfix 3.7.11); their ISO stamp
+// is not a format Postfix writes (assumptions A-12).
+//
 // The NOQUEUE line of a refusal at MAIL FROM goes on its row; a line whose
 // row is not written yet waits and is tried again; port 25's and other
 // lines are not refusals of an application.