feat(taste): phase 2b — taste_overlap candidate arm (#796)

2a re-ranks the existing pool by TasteMatch; this ensures taste-relevant tracks
ARE in the pool. Adds a 6th arm to LoadRadioCandidatesV2: in-library tracks by
the user's top positively-weighted taste-profile artists ($10 K, weight > 0,
deterministic weight-DESC,id order so it doesn't reintroduce same-day
nondeterminism). Pool-inclusion only (sim_score 0) — TasteMatch already scores
the fit. Empty for cold-start users (no profile).

- CandidateSourceLimits.TasteOverlap; default 20 (radio), 80 for For-You via
  systemForYouSourceLimits.
- You-might-like deliberately sets TasteOverlap=0: it surfaces NOT-actively-
  engaged artists, so flooding its pool with top-taste (mostly already-played)
  artists would just feed the read-time dedup.
- Test: positive-weight artist's track enters via the arm; negative-weight one
  is excluded (weight > 0). Existing pool tests unaffected (no profile seeded).

Deferred within 2b: profile-seeded For-You — marginal given the arm + TasteMatch
already inject taste broadly (top-played seed ≈ top-taste artist).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/playlists/system.go

@@ -329,6 +329,10 @@ func systemForYouSourceLimits() recommendation.CandidateSourceLimits {
 		TagOverlap:    60,
 		LikesOverlap:  40,
 		RandomFill:    150,
+		// For-You / You-might-like are the taste-driven surfaces, so pull a
+		// deep slice of the user's top taste-profile artists into the pool
+		// (#796 phase 2b). Empty for cold-start users (no profile yet).
+		TasteOverlap: 80,
 	}
 }
 

internal/playlists/you_might_like.go

@@ -89,9 +89,15 @@ func buildYouMightLike(
 	}
 
 	zeroVec := recommendation.SessionVector{Seed: true}
+	// You-might-like surfaces in-library artists the user does NOT actively
+	// engage with, so it deliberately skips the taste_overlap arm — that arm
+	// pulls top-taste (mostly already-played) artists, which would crowd the
+	// pool with entities the read-time dedup then strips. For-You/radio keep it.
+	ymlLimits := systemForYouSourceLimits()
+	ymlLimits.TasteOverlap = 0
 	cands, err := recommendation.LoadCandidatesFromSimilarity(
 		ctx, q, userID, seed, 1, zeroVec,
-		[]pgtype.UUID{seed}, systemForYouSourceLimits(),
+		[]pgtype.UUID{seed}, ymlLimits,
 	)
 	if err != nil {
 		logger.Warn("you-might-like: candidate load failed; skipping",