Pure aggregation per spec §6: Seed flag (true when prior < 3),
deduplicated Artists ordered by first appearance, Tags bag-of-counts
from tracks.genre (empty genres skipped), RecentTrackIDs preserving
input order. JSON round-trip verified.
ListRecentSessionTracks + UpdatePlayEventVector for the vector capture
path inside RecordPlayStarted. LikeTrack switches to :execrows so the
contextual-likes capture can detect insert vs already-exists. Existing
callers updated to ignore the count for now; later tasks consume it.
Composes Score over a candidate slice, sorts descending, truncates.
Pure — no IO. Liked-rank-higher and high-skip-ranks-last behaviors
are stable across RNG seeds (jitter band is smaller than LikeBoost
and SkipPenalty).
Implements spec §6 weighted-shuffle scoring without the
contextual_match_score term (sub-plan #3 adds it). Pure Go, no DB
dependency; injectable RNG for deterministic tests. Coverage 100%
on score.go via the boundary tests.
bvandeusen