[Feat]: Add parallel nodes

### Describe the feature / 功能描述

在流程图中，想实现指定节点向右布局，已有的布局不能满足
期望增加平行节点


### Are you willing to contribute? / 是否愿意参与贡献？

✅ Yes / 是

The issue provides a Chinese description and an image, but names no file, test, or entry point. Start by locating G6’s existing layout implementation and clarifying how rightward placement and “parallel nodes” should behave; done should be defined by an agreed layout example and corresponding coverage.

Written by the indexing model from the issue text.