feat(playlists): For You composition v2 — multi-seed blend + weighted fresh tail

Two approved composition changes (#1269), mechanism only — the
taste/fresh share stays data-decided (#1252) and pick_kind
attribution is unchanged.

Multi-seed blending: each day's build now seeds from up to 3 of the
user's top-5 tracks (pickDailySeeds, the generalized daily shuffle)
instead of one rotating anchor, so the mix spans neighborhoods within
a day and stops feeling bipolar as the rotation swings between
dissimilar seeds. Per-seed pools merge first-seen-deduped; the head
is filled best-first under 50/30/20 per-seed quotas (60/40 for two
seeds) so one neighborhood can't monopolize it, with thin-seed quota
spilling best-first.

Score-weighted fresh tail: the tail sample (rank 2*headN onward) was
uniform — the 380th-best candidate as likely as the 101st. It now
uses deterministic Efraimidis-Spirakis keys with weight halving every
50 ranks, so freshness keeps its "you'll probably enjoy this" half
while still rotating daily.

The retired single-seed picker's one other caller, You-might-like,
moves to pickDailySeeds(n=1) — a single neighborhood per day is right
for a short shelf, and the behavior note is inline.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TsF3cNoKrqCYsU78cXC8U6

internal/playlists/seed_selection_test.go

@@ -96,60 +96,65 @@ func TestUserIDHash_DifferentUserChangesHash(t *testing.T) {
 	}
 }
 
-// pickForYouSeedForDay rotates the chosen For-You seed across the
-// user's top-played candidates using userIDHash. Verifies the picker
-// is deterministic within a day, varies across days, and degrades
-// gracefully when fewer than 5 candidates exist.
+// pickDailySeeds shuffles the candidate pool daily-deterministically
+// and takes up to n — For-You uses n=forYouSeedCount for its
+// multi-seed blend (#1269), Songs-like uses n=3 via the
+// pickSeedArtistsForDay wrapper. Verifies determinism within a day,
+// variation across days, and graceful degradation on small pools.
 
-func TestPickForYouSeedForDay_DeterministicWithinDay(t *testing.T) {
+func TestPickDailySeeds_DeterministicWithinDay(t *testing.T) {
 	u := pgtype.UUID{Bytes: [16]byte{1}, Valid: true}
-	seeds := []pgtype.UUID{
+	pool := []pgtype.UUID{
 		{Bytes: [16]byte{10}, Valid: true},
 		{Bytes: [16]byte{20}, Valid: true},
 		{Bytes: [16]byte{30}, Valid: true},
 		{Bytes: [16]byte{40}, Valid: true},
 		{Bytes: [16]byte{50}, Valid: true},
 	}
-	a := pickForYouSeedForDay(seeds, u, "2026-05-04")
-	b := pickForYouSeedForDay(seeds, u, "2026-05-04")
-	if a != b {
-		t.Fatalf("same day should pick same seed; got %v then %v", a, b)
+	a := pickDailySeeds(pool, u, "2026-05-04", 3)
+	b := pickDailySeeds(pool, u, "2026-05-04", 3)
+	if len(a) != 3 || len(b) != 3 {
+		t.Fatalf("lens = %d, %d; want 3, 3", len(a), len(b))
+	}
+	for i := range a {
+		if a[i] != b[i] {
+			t.Fatalf("same day should pick same seeds; got %v then %v", a, b)
+		}
 	}
 }
 
-func TestPickForYouSeedForDay_VariesAcrossDays(t *testing.T) {
+func TestPickDailySeeds_VariesAcrossDays(t *testing.T) {
 	u := pgtype.UUID{Bytes: [16]byte{1}, Valid: true}
-	seeds := []pgtype.UUID{
+	pool := []pgtype.UUID{
 		{Bytes: [16]byte{10}, Valid: true},
 		{Bytes: [16]byte{20}, Valid: true},
 		{Bytes: [16]byte{30}, Valid: true},
 		{Bytes: [16]byte{40}, Valid: true},
 		{Bytes: [16]byte{50}, Valid: true},
 	}
-	picks := map[[16]byte]bool{}
+	firstPicks := map[[16]byte]bool{}
 	for i := 1; i <= 30; i++ {
 		date := "2026-05-" + twoDigits(i)
-		picks[pickForYouSeedForDay(seeds, u, date).Bytes] = true
+		firstPicks[pickDailySeeds(pool, u, date, 3)[0].Bytes] = true
 	}
-	if len(picks) < 2 {
-		t.Errorf("expected >=2 distinct seeds across 30 days; got %d", len(picks))
+	if len(firstPicks) < 2 {
+		t.Errorf("expected >=2 distinct lead seeds across 30 days; got %d", len(firstPicks))
 	}
 }
 
-func TestPickForYouSeedForDay_SingleCandidate(t *testing.T) {
+func TestPickDailySeeds_SmallPool(t *testing.T) {
 	u := pgtype.UUID{Bytes: [16]byte{1}, Valid: true}
 	only := pgtype.UUID{Bytes: [16]byte{99}, Valid: true}
-	got := pickForYouSeedForDay([]pgtype.UUID{only}, u, "2026-05-04")
-	if got != only {
-		t.Errorf("single-seed pool should return that seed; got %v", got)
+	got := pickDailySeeds([]pgtype.UUID{only}, u, "2026-05-04", 3)
+	if len(got) != 1 || got[0] != only {
+		t.Errorf("single-entry pool should return just that entry; got %v", got)
 	}
 }
 
-func TestPickForYouSeedForDay_EmptyPool(t *testing.T) {
+func TestPickDailySeeds_EmptyPool(t *testing.T) {
 	u := pgtype.UUID{Bytes: [16]byte{1}, Valid: true}
-	got := pickForYouSeedForDay(nil, u, "2026-05-04")
-	if got.Valid {
-		t.Errorf("empty pool should return zero UUID; got %v", got)
+	if got := pickDailySeeds(nil, u, "2026-05-04", 3); len(got) != 0 {
+		t.Errorf("empty pool should return nothing; got %v", got)
 	}
 }