Proposal: RichEditBox Math Mode highlight the scope of the caret in the equation

# Proposal: RichEditBox Math Mode highlight the scope of the caret

## Summary

Enhance the experience of editing complex equations in the math mode of the `RichEditBox` control by highlighting the caret's scope.

## Rationale

- Editing complex equations can be confusing without clear visual cues about the scope currently being edited.
- By improving the visual clarity of caret positioning, users can navigate and modify mathematical expressions more efficiently.

## Scope

| Capability | Priority |
| :---------- | :------- |
| Highlight the immediate scope of the caret with a dark color | Must |
| Highlight the parent scope of the caret with a light color | Must |
| Allow developers to turn on/off the highlight colors | Could |
| Allow developers to customize the highlight colors | Could |

## Important Notes

This feature exists in OneNote and other Office applications (now rebranded as Microsoft 365 Copilot), which significantly improves the experience of editing complex equations. Highlight in OneNote is shown below:


## Open Questions

The proposal names RichEditBox math mode but no implementation files, tests, or entry points. Start by reviewing the control's existing math-mode behavior, then resolve the open design questions around immediate and parent-scope highlighting and color options; done requires an agreed implementation scope.

Written by the indexing model from the issue text.