milter: the sender check keeps two From: and two Sender: values (audit M5)

Security audit 2026-10-17, finding M5 (low), stage 0. senderSession.Header
appended every From:/Sender: value; Postfix bounds one header at 60 000
bytes but not their number, and the signer's 1 MiB cap runs after the
sender check, so an application could make it hold megabytes per
connection (the reader's test: a hundred lines, 6 MB). judge needs only
none / one / more, so two of each are kept; the refusal text joins what is
kept, as before.

Tests: "a hundred From: lines" in the judgement table (refused, more than
one From: line); TestSenderKeepsTwoFromAndSenderLines. go test
./internal/milter.

Docs: security.md (the sentence "a few headers each"); CHANGELOG; plan.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/milter/sender.go

@@ -309,11 +309,18 @@ func (s *senderSession) RcptTo(rcpt string, m *milter.Modifier) (milter.Response
 
 func (s *senderSession) Header(name, value string, m *milter.Modifier) (milter.Response, error) {
 	// The name is trimmed: the obsolete syntax allows blanks before the colon.
+	// Two of each are kept: the judgement needs only none, one or more, and
+	// Postfix bounds a header's length but not their number (audit
+	// 2026-10-17, M5: a hundred 60 000-byte From: lines were 6 MB held).
 	switch textproto.CanonicalMIMEHeaderKey(strings.TrimSpace(name)) {
 	case "From":
-		s.froms = append(s.froms, value)
+		if len(s.froms) < 2 {
+			s.froms = append(s.froms, value)
+		}
 	case "Sender":
-		s.senders = append(s.senders, value)
+		if len(s.senders) < 2 {
+			s.senders = append(s.senders, value)
+		}
 	case "Subject":
 		if s.subject == "" {
 			s.subject = mailhdr.DecodeSubject(value)