Write UI Module Readme

Overview

Write module README files for packages/ui/src/lib/* as practical usage guides for agents and developers.

Optimize for fast, correct adoption:

• show the canonical usage first
• explain each exported module.* value briefly
• document the important behavior guarantees
• avoid implementation-history dumps and internal type walkthroughs

These are module docs for UI primitives, not package-level READMEs.

Workflow

1. Read the module source first.