message: Allow const batch references for sink handlers
diff: @@ -82,6 +82,14 @@ struct stream_sink_trait<void(State&, std::vector<In>&)>
  using process = detail::stream_sink_trait_invoke_all;
  };
  +/// Specializes the trait for batch-wise processing with const references.
  +template <class State, class In>
  +struct stream_sink_trait<void(State&, const std::vector<In>&)>
  + : stream_sink_trait_base<State, In> {
  + /// Defines a helper for dispatching to the processing function object.
  + using process = detail::stream_sink_trait_invoke_all;
  +};
  +
  // -- convenience alias --------------------------------------------------------
  /// Derives a sink trait from the signatures of Fun and Fin.
  