Split / Merge Styled Text Ranges

Question

## Problem

Given text with styled ranges (e.g., bold from 2-5), implement applying a new style over a range, splitting/merging adjacent ranges as needed.

## Example
```
apply bold to [3,7] -> merge/split existing style runs
```

## Constraints
- Ranges must stay non-overlapping and minimal

## What Figma looks for
Reported Figma question. Careful interval splitting/merging; keep runs canonical.