fix: challenge nonce must be 32 bytes per YAW/2 §5.1

The anchor was sending a 16-byte nonce. The spec (yaw2.0-protocol.md §5.1
and yaw2-implementation.md §7.1) requires 32 bytes. Any spec-conformant
peer would fail join signature verification against this anchor, breaking
interop. Signature verification was already using the raw nonce bytes
(not hex), so only the allocation size needed changing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/anchor/main.go

@@ -155,8 +155,8 @@ func (a *anchor) handleWS(w http.ResponseWriter, r *http.Request) {
 	ctx, cancel := context.WithCancel(r.Context())
 	defer cancel()
 
-	// Send a challenge nonce immediately.
-	nonce := make([]byte, 16)
+	// Send a challenge nonce immediately. §5.1 requires 32 bytes.
+	nonce := make([]byte, 32)
 	rand.Read(nonce)
 	nonceHex := hex.EncodeToString(nonce)
 	if err := wsjson.Write(ctx, conn, proto.AnchorMessage{