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/fileid_unix.go

@@ -9,8 +9,9 @@ import (
 )
 
 // fileID names the file itself - its inode - which a rename keeps and a new
-// file does not share. It is part of the log's fingerprint, so two log files
-// whose first bytes happen to match are still told apart. The device number
+// file does not share (rename(2): the file is moved, other hard links to it
+// are unaffected; st_ino in stat(2)). It is part of the log's fingerprint, so
+// two log files whose first bytes happen to match are still told apart. The device number
 // is left out: it can change across a host's reboot (LVM, btrfs, network
 // file systems), and the same log would then pass for a rotated one (review
 // of stage 2b).