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/machine_test.go

@@ -9,7 +9,9 @@ import (
 
 // fakeProc writes a /proc-shaped directory the sampler can be pointed at, so
 // the parsing and the arithmetic are tested against known counters instead of
-// whatever the machine running the tests happens to be doing.
+// whatever the machine running the tests happens to be doing. The file
+// contents in this test are composed, in the layouts proc(5) gives for
+// /proc/stat, /proc/meminfo and /proc/net/dev.
 func fakeProc(t *testing.T, stat, meminfo, loadavg, netdev string) string {
 	t.Helper()
 	dir := t.TempDir()