docs: public MessagePacker addPayload(byte[] src, int off, int len)
              throws IOException
      {
          if (buffer == null || buffer.size() - position < len || len > bufferFlushThreshold) {
              flush();  // call flush before add
              // Directly add the payload without using the buffer
              out.add(src, off, len);
              totalFlushBytes += len;
          }
          else {
              buffer.putBytes(position, src, off, len);
              position += len;
          }
          return this;
      }