radix-ui/primitives ? reverse-engineered prompt

Build me an open source React component library for accessible, low level UI building blocks that teams can use as the base for a design system or drop into an app piece by piece.

I want the components to feel polished, flexible, and easy to customize, with a strong focus on keyboard support, screen reader friendliness, and good developer experience. Include the usual building blocks people need for modern interfaces, plus clear docs, examples, and a simple way to test the components visually and with automated checks.

Make it feel like a real production ready library, not just a demo. If you need to, look up current docs or best practices online before filling in gaps.