[POC] Add message-available callback to CommandQueue

[mfazekas]

POC — looking for feedback, not intended to merge as-is.

Adds setMessageAvailableCallback() to CommandQueue so the server can notify the client immediately when response messages are written, instead of waiting for the next render loop poll. This is the symmetric counterpart to m_commandConditionVariable.notify_one() that already exists for client→server.

Intended use: iOS runtime would use this to dispatch_async a processMessages() call, eliminating up to 1 frame of latency on async reads.

  _commandQueue->setMessageAvailableCallback([self]() {
      // Server thread just wrote messages — wake the main thread to process them
      dispatch_async(dispatch_get_main_queue(), ^{
          [self processMessages];
      });
  });