cat: add colorization functionality

[Alonely0]

This PR adds an Easter egg to cat that colorizes the output in a gradient in homage to the notorious lolcat, accessible via the --lol flag. This feature has been implemented as a wrapper over the default writer used only to print the contents of each line, ensuring compatibility with other flags or input types. Furthermore, it takes care of not breaking detectable escape sequences and automatically detects the color space of the terminal used; these are both added functionality and/or improvements over the Rust lolcat implementation, which does not handle ANSI 8-bit color space and has trouble with some obscure flags.

Here is a video demonstration, heavily compressed by GitHub for your (in)convenience:

Screencast.From.2025-09-25.02-36-52.mp4

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)

[cakebaker]

While it looks nice, I don't see the benefit of adding such a feature.

[Alonely0]

While it looks nice, I don't see the benefit of adding such a feature.

Yeah, it's silly, but this is what makes Open Source fun and with a soul; I did DM Sylvestre first, and he was chill with adding this. I guess the main benefit is that this implementation is much faster and reliable than the original, mostly because uutils' cat is quite well implemented, and I could easily build on top of that. One example of a big improvement due to performance is that, on my machine, lolcat could not keep up with the output rate of sl (nor a few escape codes), resulting in the train losing its lower half.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

[codspeed-hq]

CodSpeed Performance Report

Merging #8741 will not alter performance

_{Comparing Alonely0:main (1cffdcb) with main (5efafd6)}

Summary

✅ 55 untouched
⏩ 1 skipped¹

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[Alonely0]

Well, if CI likes this last commit, I consider this finished. Let me know if you would like any changes done, or if you would prefer me to squash this into a single commit or to rebase to latest main before merging it.

EDIT: Should I bother fixing the cspell errors?

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)