The stored similar_artists rows key the similar artist by the SEED's source/db id, not its name,
so rank_recommended_artists can't consume them directly. group_similars_by_seed resolves each
row's source id to a seed name via a caller-supplied id_to_name map and reshapes to the
{seed_name: [{'name': similar}]} the ranker wants — the fragile id->name join, now pure + tested
(dataclass + dict rows, unknown-id drop, non-seed drop, group->rank end-to-end). 15 tests total.
c21031b9bc