Add MuCSS layout function

[pydanny]

What

μCSS is a fork of pico that adds critical features. Plus many more themes.

Reviewer Focus

Does this work?

Checklist

• Diff contains only changes for this task — no unrelated refactoring or cleanup
• Addresses exactly one issue or feature
• New or changed behavior has test coverage
• This is the simplest viable approach
• AI provenance section removed or accurate