feat: add React federation example apps

[Nsttt]

What changed

• add apps/react with one Astro host and one Astro remote using React
• add root dev:react / build scripts and a small port cleanup helper
• wire the host to own @astrojs/react, react, and react-dom explicitly
• update @module-federation/astro SSR local remote resolution to include .tsx and .jsx
• bump @module-federation/vite to ^1.13.6
• document the new React example and the host-owned React setup

Why

The repo only had the Astro-to-Astro example. We needed a concrete React remote example that matches the current package contract: no renderer API in @module-federation/astro, host config owns the React renderer setup.

The SSR local remote resolver also skipped .tsx / .jsx, which broke the React remote SSR path when ssr.localRemotes was configured.

Impact

• contributors now have a runnable host/remote React example under apps/react
• host SSR works through ssr.localRemotes for React component sources
• package examples/docs match the intended API direction

Validation

• pnpm --filter @module-federation/astro test
• pnpm --filter react-remote build
• pnpm --filter react-host build
• pnpm dev:react
• curl -I http://localhost:4331/
• curl -I http://localhost:4332/mf-manifest.json