test: regression for H4 data-plane content confidentiality

pkg/membership TestRequireEncryptedRoomsRejectsCleartext: cleartext create ->
403, encrypted -> 201, flag off -> cleartext allowed again.

pkg/client TestAudit_NoSubjectACL: under the public posture a ModeNATS room is
refused; bob (member) decrypts the secret; eve raw-subscribes to the subject off
the data plane and receives only ciphertext (non-empty AEAD nonce, no plaintext
substring) — closing the auditor's 'eve reads internal: salary numbers'.

pkg/membership/require_e2e_test.go

@@ -0,0 +1,46 @@
+package membership
+
+import (
+	"bytes"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// TestRequireEncryptedRoomsRejectsCleartext is the control-plane half of the
+// audit H4 minimum defense: with RequireEncryptedRooms on (the public posture),
+// creating a cleartext (ModeNATS) room is refused 403, while an encrypted room is
+// created normally. This is what guarantees no message ever rides the un-ACL'd
+// NATS subject in the clear on a public deployment.
+func TestRequireEncryptedRoomsRejectsCleartext(t *testing.T) {
+	srv := dosServer(t, AuthOff)
+	srv.RequireEncryptedRooms = true
+
+	create := func(encrypt bool) int {
+		body, _ := json.Marshal(createRoomReq{
+			Subject:       "payroll.subject",
+			Policy:        policyJSON{Encrypt: encrypt, Persist: encrypt, SignMsgs: encrypt},
+			Owner:         endpointJSON{Endpoint: "owner-ep", SignPub: []byte("sp"), KexPub: []byte("kp")},
+			SealedKeySelf: []byte("sealed"),
+		})
+		rec := httptest.NewRecorder()
+		srv.ServeHTTP(rec, httptest.NewRequest(http.MethodPost, "/rooms", bytes.NewReader(body)))
+		return rec.Code
+	}
+
+	// Error path: a cleartext room is refused.
+	if code := create(false); code != http.StatusForbidden {
+		t.Fatalf("cleartext room under RequireEncryptedRooms should be 403, got %d", code)
+	}
+	// Golden: an encrypted room is created.
+	if code := create(true); code != http.StatusCreated {
+		t.Fatalf("encrypted room should be 201, got %d", code)
+	}
+
+	// Edge: with the flag OFF (loopback/dev), cleartext rooms are allowed again.
+	srv.RequireEncryptedRooms = false
+	if code := create(false); code != http.StatusCreated {
+		t.Fatalf("cleartext room with the flag off should be 201, got %d", code)
+	}
+}