Group the same story across outlets into clusters, the velocity signal for
ranking. Runs automatically after 'scrape' (recompute-from-scratch, idempotent).
- internal/cluster: connected-components engine (union-find). Two articles join
when, within a time window (Cluster.WindowHours, default 72h), EITHER their
normalized-title token-set Jaccard >= threshold (default 0.6) AND they are from
different outlets, OR they share >= 1 CVE (source-agnostic). The cross-outlet
guard on the title edge kills a real false positive where two distinct CISA
advisories with identical boilerplate titles were merging. Cluster key = the
earliest article by time; SourceCount tracks distinct outlets so the reported
'multi-source' metric is truthful.
- Shared-CVE join reads article_cves, which is empty until M3 CVE extraction, so
that path is a natural no-op today and lights up in M3 with no rework.
- store: ClusterCandidates (effective time = published_at else fetched_at, bounded
by lookback), ArticleCVEMap, ReplaceClusters (single atomic tx: wipe + rebuild).
- config: Cluster.LookbackHours (default 168h) bounds the corpus for O(n^2);
validated as >= window_hours so window edges are never silently dropped.
- Proven live: 215 real articles -> 137 clusters, 2 genuinely cross-outlet
(FortiBleed x3, Scattered Spider x2). 17 cluster tests; suite offline + -race.
One read-only audit agent run; findings (multi-source stat, lookback guard) fixed in-phase.
Clustering is recompute-from-scratch each scrape; safe to re-run.
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