Multichild stages

Concept structure described as so:

Instead of linear stage execution, where

a -> b -> c -> d ->,

we want to insert support for multipath stage execution, where you can have

a -> b -> c -> d
b -> (desync) x -> y -> z

or a -> b(x -> y -> z) -> c -> d

or even a (choice)-> b
-> c
-> d

Obviously any implementation of this would be a huge breakthrough for our sister project Komponent

该 issue 从概念上描述了 multipath 和基于选择的 stage 执行，但没有列出任何文件、测试或入口点。在定位相关代码并定义如何验证完成情况之前，请先明确预期的 stage 模型、验收标准和实现范围。

由索引模型根据 Issue 内容生成。