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/recommendation/candidates.go

@@ -72,6 +72,10 @@ type CandidateSourceLimits struct {
 	TagOverlap    int
 	LikesOverlap  int
 	RandomFill    int
+	// TasteOverlap (#796 phase 2b): tracks by the user's top positively-
+	// weighted taste-profile artists. 0 disables the arm (e.g. cold-start
+	// users have an empty profile, so it contributes nothing anyway).
+	TasteOverlap int
 }
 
 // DefaultCandidateSourceLimits returns the v1 hardcoded constants per spec.
@@ -82,6 +86,7 @@ func DefaultCandidateSourceLimits() CandidateSourceLimits {
 		TagOverlap:    20,
 		LikesOverlap:  20,
 		RandomFill:    30,
+		TasteOverlap:  20,
 	}
 }
 
@@ -113,6 +118,7 @@ func LoadCandidatesFromSimilarity(
 		Limit_3: int32(limits.TagOverlap),
 		Limit_4: int32(limits.LikesOverlap),
 		Limit_5: int32(limits.RandomFill),
+		Limit_6: int32(limits.TasteOverlap),
 	})
 	if err != nil {
 		return nil, err

internal/recommendation/candidates_v2_test.go

@@ -262,6 +262,61 @@ func TestLoadCandidatesFromSimilarity_DedupTakesMaxScore(t *testing.T) {
 	}
 }
 
+// TestLoadCandidatesFromSimilarity_TasteOverlapArm (#796 phase 2b): a track by
+// a positively-weighted taste-profile artist enters the pool via taste_overlap
+// even with every other arm disabled; a negatively-weighted artist's track does
+// not (the WHERE weight > 0 filter).
+func TestLoadCandidatesFromSimilarity_TasteOverlapArm(t *testing.T) {
+	f := newFixture(t, 2) // seed + 1 other, both by the fixture artist
+	seed := f.tracks[0]
+	target := f.tracks[1]
+	ctx := context.Background()
+
+	// Fixture artist gets a positive taste weight.
+	if _, err := f.pool.Exec(ctx,
+		`INSERT INTO taste_profile_artists (user_id, artist_id, weight) VALUES ($1, $2, 5.0)`,
+		f.user, seed.ArtistID); err != nil {
+		t.Fatalf("insert taste (positive): %v", err)
+	}
+
+	// A second artist with a NEGATIVE weight — its track must be excluded.
+	negArtist, _ := f.q.UpsertArtist(ctx, dbq.UpsertArtistParams{Name: "NegArtist", SortName: "NegArtist"})
+	negAlbum, _ := f.q.UpsertAlbum(ctx, dbq.UpsertAlbumParams{Title: "NegAlbum", SortTitle: "NegAlbum", ArtistID: negArtist.ID})
+	negTrack, _ := f.q.UpsertTrack(ctx, dbq.UpsertTrackParams{
+		Title: "NegTrack", AlbumID: negAlbum.ID, ArtistID: negArtist.ID,
+		FilePath: "/tmp/neg.flac", DurationMs: 180_000,
+	})
+	if _, err := f.pool.Exec(ctx,
+		`INSERT INTO taste_profile_artists (user_id, artist_id, weight) VALUES ($1, $2, -3.0)`,
+		f.user, negArtist.ID); err != nil {
+		t.Fatalf("insert taste (negative): %v", err)
+	}
+
+	// Only the taste_overlap arm is enabled.
+	limits := CandidateSourceLimits{TasteOverlap: 10}
+	got, err := LoadCandidatesFromSimilarity(
+		ctx, f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, limits,
+	)
+	if err != nil {
+		t.Fatalf("load: %v", err)
+	}
+	var sawTarget, sawNeg bool
+	for _, c := range got {
+		switch c.Track.ID {
+		case target.ID:
+			sawTarget = true
+		case negTrack.ID:
+			sawNeg = true
+		}
+	}
+	if !sawTarget {
+		t.Error("positive-taste-artist track missing (taste_overlap arm didn't contribute)")
+	}
+	if sawNeg {
+		t.Error("negative-taste-artist track present (weight > 0 filter failed)")
+	}
+}
+
 func TestLoadCandidatesFromSimilarity_EmptyLibrary_NoError(t *testing.T) {
 	f := newFixture(t, 1) // just the seed
 	seed := f.tracks[0]