Sync sitemap lastmod dates with spec entry updated field

[jdevalk]

What this changes

Implements dynamic sitemap generation that derives <lastmod> dates from the updated front matter field of spec entries, rather than using build timestamps. This ensures the sitemap stays in sync with actual content changes and matches the source of truth already used by the RSS feed.

Code changes:

• astro.config.mjs: Added buildLastmodMap() function that parses all spec entry front matter to extract updated dates and builds a canonical URL → lastmod map. Integrated into the sitemap serializer to populate <lastmod> for each URL.
• src/content/spec/seo/xml-sitemaps.md: Added a "This site ships it" section documenting that specification.website now generates sitemaps with accurate lastmod dates derived from content, not build time.

The implementation:

• Reads all .md and .mdx files under src/content/spec/
• Extracts updated, category, and slug from YAML front matter
• Skips draft entries
• Maps individual spec URLs and category index pages to their respective lastmod dates
• Sets root-level pages (/, /spec/, /checklist/) to the newest entry's date across the whole collection
• Passes the map to @astrojs/sitemap's serialize() hook to inject ISO 8601 dates

Sources

• Sitemaps.org protocol — <lastmod> specification
• W3C: Last-Modified — HTTP semantics

Checklist

• Sources cited on changed page.
• Status is honest (page documents existing implementation).
• Platform-agnostic — describes the outcome (accurate lastmod dates), not implementation details.
• npm run build passes locally.
• No accidental dependencies added (uses only Node.js built-ins and existing Astro integration).

https://claude.ai/code/session_016DgaQscUXMFQBcJX9Q1J63