Mistake: future-dated or invalid lastmod values

Mistake: future-dated or invalid lastmod values

A scheduled post leaking its publish-at timestamp into a live sitemap produces a future <lastmod>. A malformed date (no timezone, wrong order, 0000-00-00) fails validation. Either makes Google distrust every lastmod in the file.

Fix checklist:
— ✅ No lastmod is in the future relative to the file's generation time
— ✅ Format is W3C datetime: YYYY-MM-DD or full YYYY-MM-DDThh:mm:ss+00:00
— ✅ Scheduled/unpublished rows are excluded entirely until they go live
— ✅ No null, epoch-zero, or placeholder dates — omit lastmod rather than emit a bad one

Definition of done: parse every lastmod; assert all are valid W3C datetimes and none exceed the generation timestamp. One future or malformed date fails the build.