rss_generator set <lastBuildDate> to datetime.utcnow(), so every republish
rewrote it even when the digest was unchanged — leaving a 1-line develop<->master
drift in v2-docs/feed.xml (the residual after the PR Guardian drift fix). Derive
it from the digest's _meta.last_updated (the actual analysis date), falling back
to the most recent item date, so the feed is a pure function of its input:
regenerating unchanged content now yields byte-identical output. Verified
deterministic across repeated runs.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
