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/lines_test.go

@@ -9,6 +9,10 @@ import (
 	"time"
 )
 
+// The mail.log lines in this file are composed by hand. "postfix/submission/
+// smtpd" is a tag this image never writes (build/postfix-config.sh sets
+// syslog_name on port 25 only); it stands for any "-o syslog_name" listener.
+//
 // Which lines are kept for a message's page, and under which queue id: the id
 // at the head of the line and nothing the far end wrote further along it.
 func TestQueueLineHead(t *testing.T) {