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/journal.go

@@ -54,7 +54,9 @@ func listenMilter(socketPath string) (net.Listener, error) {
 		return nil, err
 	}
 	// Clear a stale socket left behind by an unclean shutdown, otherwise the
-	// listen below fails with "address already in use".
+	// listen below fails with "address already in use" (unix(7): the socket
+	// file "must be deleted by the caller when it is no longer needed"; bind(2)
+	// EADDRINUSE).
 	if err := os.Remove(socketPath); err != nil && !errors.Is(err, os.ErrNotExist) {
 		return nil, err
 	}
@@ -67,6 +69,9 @@ func listenMilter(socketPath string) (net.Listener, error) {
 	// group access. The socket inherits group `selfpost` from the setgid parent
 	// dir entrypoint.sh prepares; make it group read/write so postfix can reach
 	// it (connecting to a Unix socket needs write permission on the node).
+	// unix(7): "connecting to a stream socket object requires write
+	// permission on that socket"; inode(7): a file made in a set-group-ID
+	// directory takes the directory's group.
 	if err := os.Chmod(socketPath, 0o660); err != nil {
 		ln.Close()
 		return nil, err