### What & Why
Previously when rendering the root `/_not-found` in production, we'll always override the parallel routes component children with not-found component, this will break the navigation in build mode from root 404 `/_not-found` path.
### How
The new solution is to change the root `/_not-found` rendering strategy. Previously the loader tree of `/_not-found` look like this
```js
['',
{
children: ['not-found', {}, {}]
},
{ layout: ..., 'not-found': ...}
]
```
it's not a pretty valid tree, which could lead to problems during rendering.
New solution is to change the children to render a page, but the page content is `not-found.js` component. The new tree of root not-found will look like
```js
['',
{
children: ['__PAGE__', {}, {
page: ... // same as root 'not-found'
}]
},
{ layout: ..., 'not-found': ...}
]
```
This change could fix the navigation on the root not-found page.
Fixes#52264
