- Lead.__post_init__: raises ValueError on empty provenance list
- find_gaps: no-chunk path now carries topic-marker Provenance instead of []
- _update_index: docstring corrected (rebuilds, not append-only)
- test_model: test_lead_empty_provenance_raises covers the new invariant
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>