feat: add client kind command matrix parity

src/NATS.Server/Protocol/ClientCommandMatrix.cs

@@ -0,0 +1,17 @@
+namespace NATS.Server.Protocol;
+
+public sealed class ClientCommandMatrix
+{
+    public bool IsAllowed(ClientKind kind, string? op)
+    {
+        if (string.IsNullOrWhiteSpace(op))
+            return true;
+
+        return (kind, op.ToUpperInvariant()) switch
+        {
+            (ClientKind.Router, "RS+") => true,
+            (_, "RS+") => false,
+            _ => true,
+        };
+    }
+}