Add bullframe.css

[marcop135]

Summary

Adds bullframe.css to the Classless section.

• Semantic-by-default CSS framework with optional classless builds, shared --bf-* tokens, and system dark
• Native CSS, no Sass, no JS runtime
• Screenshots: light (bullframe-classless) and system-dark (bullframe-classless-system-default with prefers-color-scheme: dark)
• Maintainer of bullframe.css

Docs: https://bullframecss.marcopontili.com
Classless asset: https://cdn.jsdelivr.net/npm/bullframe.css@6.0.0/dist/css/bullframe-classless.min.css

Checklist

• Entry in data/projects.toml
• Screenshots + thumbnails
• README.md regenerated via render-template.ts