clientRank already existed specifically to stop stale high-scorers from
leading the feed, but computed age from updatedAt — the same timestamp
that keeps resetting whenever the backend attaches any article to a
cluster (BACKLOG #15). A fossil's age always read as ~0, silently
defeating the whole mechanism. Fixed to use firstSeenAt.
clientRank's decay is soft (max 20% penalty) and isn't always enough
against a high signalScore alone, so added StorySummary.isFossil (>5
days old by firstSeenAt) as a hard backstop: mainStories and
makeDigest now sink fossils below current stories, so one can never
take the hero/lead slot regardless of score. Still shown in the feed,
just not in the most prominent position.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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