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/health/milter.go

@@ -16,7 +16,9 @@ import (
 // add to a poll.
 const milterDialTimeout = time.Second
 
-// CheckMilter probes a milter address in the two forms Postfix takes:
+// CheckMilter probes a milter address in the two forms build/postfix-config.sh
+// lets INBOUND_ANTISPAM_MILTER take (Postfix 3.7.11, src/milter/milter8.c,
+// milter8_connect(), also accepts "local:", which that script refuses):
 // unix:/path is stat-ed like any milter socket, and inet:host:port is dialled
 // and the connection closed at once, with nothing written. required says what
 // a failure costs, as for CheckSocket: an error when mail stops without the