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.

• Identify the actual public exports and their roles.
• Confirm the behavior from code, not memory.

2. Read nearby tests and demos.

• Pull behavior notes from tests.
• Reuse the most realistic example shape from a demo when available.

3. Document the module as it exists today.

• Do not describe planned APIs.
• Do not document internal coordinators, private state, or workaround history unless they are part of the public contract.

4. Keep the README short and scannable.

• Prefer short paragraphs and flat bullets.
• Use one strong canonical example instead of multiple weak snippets.

Recommended Structure

Use this structure unless the module needs something more specific:

1. `# ModuleNa…