- Add weights.rs with configurable RelationshipWeights
- Add builder.rs with ArtistVectorBuilder for constructing vectors
- Integrate MusicBrainz relationships (members, groups, collaborators)
- Integrate Qobuz similar artists API
- Add ensure_vector() with TTL for lazy building
- Add resolve_qobuz_to_mbid() for ID resolution via name search
- 21 tests passing (sparse_vector, store, weights, builder)