Add QbzCore::generate_playlist_suggestions + a per-user artist_vectors store
field (Arc<tokio::Mutex<Option<ArtistVectorStore>>>) with async
set_artist_vectors / clear_artist_vectors. It resolves each playlist artist NAME
to a confident MusicBrainz id (reusing musicbrainz_resolve_artist; the names come
from the playlist's own Qobuz tracks so they're already canonical, making the
Tauri command's extra Qobuz-name re-fetch unnecessary), then runs the
SuggestionsEngine over the core-owned clients + vector store.
Also aligns the qbz-reco builder/engine handle types to qbz-core's reality (the
plan's by-ref pattern doesn't fit): mb_client = Arc<MusicBrainzClient> (no lock,
Send+Sync); mb_cache = Arc<std::sync::Mutex<...>> (locked only in await-free
windows); Qobuz client = Arc<RwLock<Option<QobuzClient>>> (unwrapped per access).
The vector store stays tokio::Mutex (held across awaits by the engine). No
dependency cycle (qbz-reco does not depend on qbz-core). qbz-core checks clean;
23 qbz-reco tests green.
cc
vicrodh
9cc44f5d1d