feat: render the listing pages, 404 and Atom feed with Astro

[jpoissonnet]

Preview

https://feat-astro-listings.dsvmt7xpjktgx.amplifyapp.com

Stacked on #498. Production stays on Jekyll.

Summary

• Homepage and /blog/pageN/ (10 Articles per page, same pagination and excerpts as jekyll-paginate). An excerpt is the front matter excerpt, or the Markdown before excerpt_separator (default: the first paragraph) with the Article's link and footnote definitions, like Jekyll.
• /lft/, /meetups/ and /tags/, rebuilt from the Talks, Articles and _data/topics.yml. /tags/ lists topics only; the page tags Jekyll also listed (Page, oss, open source) are dropped.
• /jobs/ and /oss/ are the pages/*.md content with a Page.astro layout.
• 404 and /feed.xml (Atom, 10 newest Articles, same ids, links, dates and summaries as jekyll-feed).
• build.format: 'preserve', so /x/ pages are written as x/index.html, and Articles and Talks still land on a/b.html.
• Entries with the same date are sorted by file path, like Jekyll, so listings match production order.
• Typography matches kramdown: -- is an en dash.
• Content: CommonMark keeps reading raw HTML after <br>, <hr> or </figure> until a blank line, reads * followed by 5 spaces as the start of a code block, and needs a header row in tables. 9 Articles get a blank line, a single space after the list marker, or an empty header row. These render the same with kramdown.

Known differences from production:

• Inline author names (old Tumblr posts) are shown, as in feat: render Article and Talk pages with Astro #498.
• GFM footnote ids (#user-content-fn-… instead of #fn:…).
• Feed titles keep straight quotes, where jekyll-feed curls them.

Test plan

• Local build: every listing link (homepage, /blog/page2/–/blog/page22/, /lft/, /meetups/, /tags/, /oss/, /jobs/) matches production, except the differences above.
• Local build: no Markdown left as literal text that production renders, over all 337 pages.
• Local build: /feed.xml is valid XML with the same entries as production.
• Screenshots on the Amplify preview: the first screen of the homepage, /blog/page2/, /lft/, /meetups/, /oss/ and an Article is pixel-identical to production; /tags/, /jobs/ and /404.html differ only by the dropped page tags and the footer.
• Preview: /lft/, /meetups/, /tags/, /oss/, /jobs/, /blog/page22/ respond with the right titles
• Preview: /404.html shows the 404 page. Unknown URLs show the homepage, because the Amplify app rewrites every 404 to /index.html; GitHub Pages serves 404.html.
• Preview: /feed.xml serves 10 entries