fix(0005b): drop unsigned frames in SignMsgs rooms (close sig-nil spoof)

client.processFrame verified a frame's signature only when one was present
(`info.Policy.SignMsgs && f.Sig != nil`). In a room whose policy REQUIRES
per-message signatures, an attacker with data-plane access could publish a raw
frame with Sig==nil and a forged Sender, and the receiver accepted it as
authentic because the verification block was skipped (audit N3, report 0006).
On a signed-but-cleartext room any peer that knows the subject could thus
impersonate any sender.

Fix: in a SignMsgs room a missing signature is itself a rejection. processFrame
now drops any frame with Sig==nil before attempting verification:

    if info.Policy.SignMsgs {
        if f.Sig == nil { return }   // signature required but absent: drop
        // verify ...
    }

Non-signed rooms (ModeNATS) are unaffected: unsigned frames there are still
delivered, so the plain-NATS path is unchanged.

Verification (pkg/client/sig_nil_spoof_test.go, TestReaudit_SigNilSpoof):
- golden: a properly signed frame from a member is delivered.
- error : an unsigned frame with a forged Sender in a SignMsgs room is dropped
  (the test fails with "SIG-NIL SPOOF: receiver accepted ..." when the fix is
  reverted, confirming it is a real regression guard).
- edge  : a non-signed room still delivers an unsigned frame.
- CGO_ENABLED=0 go build ./... && go vet ./... && go test -count=1 ./...  green.

Refs: report 0006 N3, issue 0005b.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/client/client.go

@@ -799,7 +799,17 @@ func (c *Client) processFrame(roomID string, info roomView, data []byte, handler
 	if err != nil {
 		return
 	}
-	if info.Policy.SignMsgs && f.Sig != nil {
+	// A room with SignMsgs REQUIRES a signature, so an unsigned frame is
+	// unauthenticated and must be dropped — not silently accepted. The previous
+	// `&& f.Sig != nil` guard verified the signature only when one was present, so
+	// an attacker with data-plane access could publish a frame with Sig==nil and a
+	// forged Sender and have the receiver accept it as authentic in a room that
+	// demands signatures (audit N3, report 0006). Requiring the signature first
+	// closes that spoof.
+	if info.Policy.SignMsgs {
+		if f.Sig == nil {
+			return // signature required by room policy but absent: drop
+		}
 		pub, err := c.signerPub(roomID, f.Sender)
 		if err != nil || !cs.VerifyEd25519(pub, f.SigningBytes(), f.Sig) {
 			return // unauthenticated frame: drop