Adds `checkerboard` to `Background`, and use it for the side-by-side
diff.
Note that, since the blockmap can contain multiple `Spacer`s without any
gaps
in between, we ensure that the checkerboard pattern always shows an even
integer number of squares per line, so there are no obvious
discontinuities
in the pattern when this happens.
<img width="590" height="300" alt="image"
src="https://github.com/user-attachments/assets/e8f75f90-b230-4078-bce0-cb3c15613fe7"
/>
Release Notes:
- N/A *or* Added/Fixed/Improved ...
---------
Co-authored-by: Cole Miller <cole@zed.dev>
