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>

cmd/panel/httpserver.go

@@ -99,7 +99,9 @@ func newPanel(cfg config, st *store.Store) (*web.Server, error) {
 	ms := newMailStack(cfg, st)
 	// postfix-config.sh has already run (entrypoint). postconf -h is the
 	// effective config, including a manual override; the panel keeps this
-	// snapshot for the process lifetime (architecture.md).
+	// snapshot for the process lifetime (architecture.md). Postfix 3.7.11,
+	// postconf(1): without -d or -n it shows main.cf's value or the default,
+	// and -h drops the "name = " label; values are not $-expanded without -x.
 	retryPolicy := postfix.LoadRetryPolicy()
 	return web.New(st, ms.Domains, ms.Apps, ms.Inbound, ms.Lists, ms.DMARC, web.Config{
 		Hostname:                   cfg.hostname,