chore: scaffold Astro

[jpoissonnet]

Preview

https://pr-497.dsvmt7xpjktgx.amplifyapp.com

Summary

Base of the Phase 1 stack (Astro port). Nothing changes for production, which stays on Jekyll.

• amplify.yml builds astro/ and serves astro/dist, so this branch and the ones stacked on it get Astro previews. master has no amplify.yml, so every other PR keeps building Jekyll from the Amplify app settings.
• astro/ has its own package.json and is excluded from Jekyll.
• astro/src/content.config.ts loads _articles and _talks in place, with schemas matching script/check-front-matter.py, plus _data/authors.yml. Topics are validated against _data/topics.yml.
• assets/ and images/ are copied into the build output so they keep their URLs.
• New astro job in the integration workflow, on Node 26.
• Node 26 on Amplify: the feat/astro-scaffold branch has a _CUSTOM_IMAGE=node:26 override, set on the branch in the Amplify app. Pull request previews against master still use the app-wide ruby:3.1 image, so amplify.yml installs Node 26 with nvm only when the image has none.
• _articles/2012-11-23-... had a broken image path (h/images/...), which Astro rejects. It is now /images/....

Test plan

• Both the branch deployment (Node 26 from the image) and the pr-497 preview (Node 26 from nvm) build with v26.10.0.
• The Amplify preview uses amplify.yml and serves the Astro placeholder page ("218 Articles and 119 Talks").
• The preview serves /assets/fonts/font-awesome/fa-solid-900.woff2 and an image under /images/posts/....
• The astro CI job passes next to the Jekyll build.
• npm run build locally: 337 entries, dist/ contains assets/ and images/.
• topics: [bogus] on an Article fails the Astro build.

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