feat(www): emit sitemap lastmod from content dates (#50198)

I added content dates to `sitemap_www.xml` and `dateModified` to blog
JSON-LD so crawlers can compare freshness with page metadata. Both use
`updated` when present, otherwise the publication date.

**Changed:**
- **Consistent dates:** I use the same frontmatter parser for the blog
page and sitemap. It preserves authored dates across quoting and
timezones and rejects JavaScript frontmatter.
- **Invalid dates stop the build:** I reject impossible calendar days,
out-of-range times and offsets, malformed dates, and `updated` before
publication. Content changes run the generator in CI.
- **Authoring:** I documented optional `updated` for substantive
revisions. Events, static pages, and `/evals` omit `<lastmod>`.

**Note:** Changelog dates come from RSS. Existing sitemap omissions for
nonnumeric changelog slugs (GROWTH-1212) and app-router pages
(GROWTH-1214) remain separate.

## To test

On the preview:
- [x] Open `/sitemap_www.xml`: blog, alternatives, customer stories, and
included changelog entries should carry `YYYY-MM-DD` lastmod values.
Verified on the 2092513 preview: all 425 blog, 3 alternatives, 43
customer story, and 207 changelog entries carry a `YYYY-MM-DD` lastmod,
zero malformed values. Production currently emits no lastmod at all.
- [x] Inspect `/blog/supabase-is-now-available-in-gemini-enterprise`:
BlogPosting `dateModified` should be `2026-09-09`, matching its sitemap
entry. Verified: one BlogPosting block, `datePublished` and
`dateModified` both `2026-09-09`, sitemap lastmod `2026-09-09`.
- [x] Find the `/company` and event entries in the sitemap: neither
should carry lastmod. Verified: `/company` and all 13 `/events/` entries
have no lastmod.
- [x] Added: `/evals` and the `/changelog` index carry no lastmod
either.
- [x] Added: the blog page renders with no new console errors. The only
console error is a `/docs?_rsc=` prefetch 404: the preview host serves
404 for `/docs` itself, unrelated to this change.

## Linear
- fixes GROWTH-1206


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Blog posts now support optional updated dates for substantive
revisions.
- Sitemap entries include accurate modification dates for blog,
alternatives, customers, and changelog content.
  - Blog structured data now includes the post’s modification date.

- **Bug Fixes**
- Improved validation prevents invalid or inconsistent content dates
from generating incorrect sitemap data.
- Changelog sitemap links are deduplicated and assigned their published
dates.

- **Documentation**
- Added guidance for specifying publication and update dates in blog
post metadata.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.github/workflows/www-tests.yml

@@ -8,8 +8,13 @@ on:
       - 'apps/www/next.config.mjs'
       - 'apps/www/next.config.js'
       - 'apps/www/lib/**/*.js'
+      - 'apps/www/lib/**/*.mjs'
       - 'apps/www/content/md/**'
       - 'apps/www/scripts/**/*.mjs'
+      - 'apps/www/internals/**/*.mjs'
+      - 'apps/www/_blog/**'
+      - 'apps/www/_alternatives/**'
+      - 'apps/www/_customers/**'
       - 'apps/www/public/.well-known/**'
       # www catalog tests check that linked docs guides exist, so guide changes
       # must trigger these tests and their sources must be included in checkout.