Undated listing fallback

Some news indexes contain stable story cards but no publication date. Enable the conservative fallback explicitly with --undated or undated_fallback: true. It runs only after stronger plugin, bridge, spec, and dynamic strategies return no items.

Candidates need repeated sibling/card structure and meaningful title/link pairs. Guards reject navigation, forms, product grids, and generic link collections. Accepted items keep document order, use stable link/title-derived IDs, set pubdate to null, and report undated provenance plus confidence diagnostics. Tune undated_min_items and undated_min_confidence only against a labeled local corpus; lower thresholds increase false positives.

All serializers preserve undated items: date fields are null/empty where the target format permits them and are not fabricated.