feat: add `frontend-astro` module

[ronak-create]

Summary

Add Astro as a frontend option for content-heavy and static sites.

Why

Astro is the dominant choice for documentation sites, marketing pages, and blogs.
It supports React/Vue/Svelte islands, making it a natural fit for Foundation's modular model.

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Files to generate

• src/pages/index.astro
• src/layouts/Layout.astro
• astro.config.mjs
• tsconfig.json
• public/favicon.svg

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Implementation hints

• Add packages/modules/src/frontend/astro.ts
• Key deps: astro
• Integration options (React, Vue, Svelte islands) can be wired via configPatches if those UI modules are also selected — start with the base module only
• Follow react-vite.ts as the closest structural analogue
• Register in:
  • index.ts
  • registry-loader.ts
  • graph-definition.ts

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Contributor guidance

Refer to CONTRIBUTING.md → "Adding a New Module"

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Acceptance criteria

• foundation create lists Astro as a frontend option
• Generated project runs astro dev without errors
• Snapshot test added