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/offset.go

@@ -17,7 +17,9 @@ import (
 
 const (
 	// fingerprintSize is how many bytes from the head of the log identify it.
-	// Postfix writes a timestamped line per event, so the first 512 bytes are
+	// Postfix writes a timestamped line per event (Postfix 3.7.11,
+	// src/util/msg_logger.c, msg_logger_print(): "%b %d %H:%M:%S" on every
+	// record), so the first 512 bytes are
 	// effectively unique per log generation — enough to tell "the file we were
 	// reading" from "a fresh one created by logrotate while we were down",
 	// which os.SameFile cannot answer across a restart.
@@ -128,7 +130,8 @@ func (t *tracker) resume(f *os.File) int64 {
 
 // readRotated reads, when the log was rotated while the panel was down, the
 // rest of the file the saved place belongs to: logrotate renamed it to
-// mail.log.1 (delaycompress leaves it plain), and what Postfix wrote there
+// mail.log.1 (delaycompress leaves it plain - logrotate 3.21.0, logrotate.c,
+// prerotateSingleLog()), and what Postfix wrote there
 // between the panel's stop and the rotation - deliveries, refusals, deferrals -
 // was otherwise never read (audit 2026-10-12, stage 2b). It is read from the
 // saved offset to its end, line by line through handle, before the new file