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/main.go

@@ -34,7 +34,12 @@ func main() {
 		log.SetPrefix("dmarc-ingest: ")
 		if err := runDMARCIngestMode(); err != nil {
 			log.Printf("ingest failed: %v", err)
-			os.Exit(75) // EX_TEMPFAIL — ask Postfix to defer
+			// EX_TEMPFAIL — ask Postfix to defer. Postfix 3.7.11,
+			// src/global/pipe_command.c, pipe_command(): a sysexits code maps
+			// to its DSN (src/global/sys_exits.c: 75 is "4.3.0") and a 4 defers,
+			// unless the output starts with a "D.S.N text" of its own - the
+			// log prefix above makes sure it does not.
+			os.Exit(75)
 		}
 		return
 	}
@@ -200,7 +205,14 @@ func antispamAction(raw string) string {
 
 // saslRealm chooses the realm new SASL accounts live under. It mirrors the
 // hostname Postfix's SASL layer uses so a client authenticating with a bare
-// login resolves to the right account.
+// login resolves to the right account. Postfix 3.7.11,
+// src/xsasl/xsasl_cyrus_server.c, xsasl_cyrus_server_create(): serverFQDN is
+// $myhostname (= SELFPOST_HOSTNAME, build/postfix-config.sh) and user_realm
+// NULL while smtpd_sasl_local_domain is empty; Cyrus SASL 2.1.28,
+// plugins/sasldb.c, sasldb_auxprop_lookup(), and common/plugin_common.c,
+// _plug_parseuser(): a login
+// without "@" is then looked up in the realm serverFQDN. A SASL_REALM other
+// than SELFPOST_HOSTNAME does not follow that - assumptions A-48.
 func saslRealm() string {
 	if r := os.Getenv("SASL_REALM"); r != "" {
 		return r