Add @typestyles/react package with css prop and styled API (P3.25)

[charleswallace0826]

Summary

Implements P3.25 from IMPROVEMENTS.md — a new @typestyles/react package for teams migrating from Emotion/styled-components.

• css prop — Maps inline CSSProperties to styles.hashClass via two paths:
  • Runtime: @typestyles/react/jsx-runtime + TypeStylesProvider and resolveCssPropClass
  • Build-time: @typestyles/react/babel plugin transforms static css={{…}} props to className
• createStyled(styles) — Thin React wrapper over styles.component with ComponentVariants typing, variant prop splitting, and cx class merging
• Monorepo integration — Package scaffold (README, CHANGELOG, changeset), root react script, IMPROVEMENTS checkbox marked complete

No CSS generation is duplicated — all styling delegates to existing typestyles APIs.

Test plan

• pnpm install
• pnpm --filter typestyles build && pnpm --filter @typestyles/eslint-plugin build
• pnpm --filter @typestyles/react build
• pnpm --filter @typestyles/react test
• pnpm --filter @typestyles/react typecheck
• pnpm --filter @typestyles/react lint

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