feat(ui): add component metadata contracts

[artiphishle]

Summary

Implements #46.

Adds provider-neutral UI component metadata contracts for:

• component categories
• component prop schemas and prop values
• array item prop schemas
• blueprints and default props
• i18n metadata
• event metadata and event payload field metadata
• slot metadata
• component metadata registries
• package-level component manifests for extension packages

Also adds:

• root src/index.ts export
• ./ui package subpath export
• serialization tests for component metadata, registries, and extension package manifests
• minor changeset

Scope intentionally excluded

• no ZORA migration in this PR
• no ZORA compatibility aliases in this PR
• no bindable metadata extension work from 03 — Add shared bindable component metadata contracts #49
• no Studio UI changes
• no Runtime changes

Verification

I could not run local validation from this session because the environment cannot reliably clone/resolve GitHub repositories.

Please run before merge:

bun install
bun run build
bun run lint:fix
bun run test

Notes

The new metadata contracts are type-only and independent from React, React Native, ZORA, Surface, Studio, and Runtime implementations.