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

The issue describes multipath and choice-based stage execution conceptually, but names no files, tests, or entry points. Clarify the intended stage model, acceptance criteria, and implementation scope before locating the relevant code and defining how completion will be verified.

Written by the indexing model from the issue text.