feat(playlists): daily seed rotation + jitter + 12+13 split for system playlists

Five diversity mechanics — applied to both For-You and Songs-Like-X.

1. For-You seed rotates daily across the user's top-5 most-played
   tracks. pickForYouSeedForDay uses userIDHash(user, day) mod
   len(seeds) so today's mix uses an entirely different similarity
   pool than tomorrow's. Within-day determinism preserved.
2. JitterMagnitude bumped 0.0 → 0.1. The scoring RNG is now seeded
   by userIDHash(user, day) rather than the no-op, so near-tied
   candidates shuffle daily without breaking within-day stability.
3. Head/tail split moves from 20+5 to 12+13. Roughly half the
   playlist comes from the tail now (daily-deterministic via
   tieBreakHash), giving the user substantially different content
   while a 12-track anchor of strong similarity matches keeps the
   mix recognizable.
4. Songs-Like-X seed artists shuffle daily across the user's top-5
   played artists. pickSeedArtistsForDay applies a userIDHash-seeded
   Fisher-Yates and takes 3.
5. scoreAndSortCandidates / pickTopN / pickHeadAndTail gain a userID
   parameter so the RNG can be seeded per-user; existing call sites
   updated; noopRNG removed.

Test fixtures widened similarity gaps (e.g. float64(50-i) instead of
(50-i)/50) so the new jitter (±0.1) doesn't perturb head ordering in
assertions about the head/tail mechanism. New seed_selection_test
coverage for userIDHash + pickForYouSeedForDay + pickSeedArtistsForDay
spans deterministic-within-day, varies-across-days, and graceful
degradation with small candidate pools.

PickTopPlayedTrackForUser replaced by PickTopPlayedTracksForUser
:many in the prior commit (b4801c2). The For-You seed lookup now
goes through pickForYouSeedForDay over the returned slice.
PickSeedArtists's LIMIT widened to 5 in the same prior commit.

For #392 Half A — system playlist content diversity. Half B
(per-user timezone scheduling) is a separate spec.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/playlists/foryou_test.go

@@ -10,11 +10,20 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/recommendation"
 )
 
+// testUserID is a stable UUID used to seed the daily-determinism RNGs in
+// scoreAndSortCandidates / pickHeadAndTail / pickTopN. Tests pass it to
+// every call so the jitter is reproducible across test runs.
+var testUserID = pgtype.UUID{Bytes: [16]byte{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16}, Valid: true}
+
 // makeCand constructs a Candidate with distinct track/album/artist IDs
 // and a SimilarityScore that drives distinguishable scores when passed
 // through recommendation.Score. Using SimilarityScore alone is sufficient
 // because systemMixWeights.SimilarityWeight = 1.5 (non-zero), so
 // different similarity values produce different scores.
+//
+// Tests below use similarity values with gaps wide enough that the
+// systemMixWeights JitterMagnitude (±0.1) cannot perturb the top-of-pool
+// ordering, so assertions about head order remain meaningful.
 func makeCand(trackN, albumN, artistN int, similarity float64) recommendation.Candidate {
 	var tID, aID, arID pgtype.UUID
 	tID.Valid, aID.Valid, arID.Valid = true, true, true
@@ -90,7 +99,7 @@ func TestPickHeadAndTail_SmallPool(t *testing.T) {
 		makeCand(2, 11, 101, 0.9),
 		makeCand(3, 12, 102, 0.8),
 	}
-	got := pickHeadAndTail(in, "2026-05-07", time.Now(), 5, 2)
+	got := pickHeadAndTail(in, testUserID, "2026-05-07", time.Now(), 5, 2)
 	if len(got) != 3 {
 		t.Errorf("len = %d, want 3 (pool too small for head/tail split)", len(got))
 	}
@@ -98,11 +107,12 @@ func TestPickHeadAndTail_SmallPool(t *testing.T) {
 
 func TestPickHeadAndTail_ExactlyTotal(t *testing.T) {
 	// Pool == headN+tailN: no tail to sample from, returns all.
+	// Wide score gaps (1.0 per step) keep jitter from perturbing order.
 	in := make([]recommendation.Candidate, 0, 7)
 	for i := 0; i < 7; i++ {
-		in = append(in, makeCand(i+1, i+1, i+1, float64(7-i)/10.0))
+		in = append(in, makeCand(i+1, i+1, i+1, float64(7-i)))
 	}
-	got := pickHeadAndTail(in, "2026-05-07", time.Now(), 5, 2)
+	got := pickHeadAndTail(in, testUserID, "2026-05-07", time.Now(), 5, 2)
 	if len(got) != 7 {
 		t.Errorf("len = %d, want 7 (pool == total)", len(got))
 	}
@@ -111,11 +121,12 @@ func TestPickHeadAndTail_ExactlyTotal(t *testing.T) {
 func TestPickHeadAndTail_HeadAndTailSplit(t *testing.T) {
 	// Pool of 100 distinct (album, artist) pairs; no caps trim.
 	// With headN=20, tailN=5, expect 25 entries total.
+	// Wide score gaps (1.0 per step) keep jitter from perturbing order.
 	in := make([]recommendation.Candidate, 0, 100)
 	for i := 0; i < 100; i++ {
-		in = append(in, makeCand(i+1, i+1, i+1, float64(100-i)/100.0))
+		in = append(in, makeCand(i+1, i+1, i+1, float64(100-i)))
 	}
-	got := pickHeadAndTail(in, "2026-05-07", time.Now(), 20, 5)
+	got := pickHeadAndTail(in, testUserID, "2026-05-07", time.Now(), 20, 5)
 	if len(got) != 25 {
 		t.Errorf("len = %d, want 25 (20 head + 5 tail)", len(got))
 	}
@@ -125,11 +136,11 @@ func TestPickHeadAndTail_Determinism(t *testing.T) {
 	// Same inputs, same dateStr → identical output both times.
 	in := make([]recommendation.Candidate, 0, 100)
 	for i := 0; i < 100; i++ {
-		in = append(in, makeCand(i+1, i+1, i+1, float64(100-i)/100.0))
+		in = append(in, makeCand(i+1, i+1, i+1, float64(100-i)))
 	}
 	now := time.Now()
-	got1 := pickHeadAndTail(in, "2026-05-07", now, 20, 5)
-	got2 := pickHeadAndTail(in, "2026-05-07", now, 20, 5)
+	got1 := pickHeadAndTail(in, testUserID, "2026-05-07", now, 20, 5)
+	got2 := pickHeadAndTail(in, testUserID, "2026-05-07", now, 20, 5)
 	if len(got1) != len(got2) {
 		t.Fatalf("len mismatch: %d vs %d", len(got1), len(got2))
 	}
@@ -147,11 +158,11 @@ func TestPickHeadAndTail_HeadStable_TailVariesAcrossDays(t *testing.T) {
 	// because tieBreakHash incorporates the date.
 	in := make([]recommendation.Candidate, 0, 100)
 	for i := 0; i < 100; i++ {
-		in = append(in, makeCand(i+1, i+1, i+1, float64(100-i)/100.0))
+		in = append(in, makeCand(i+1, i+1, i+1, float64(100-i)))
 	}
 	now := time.Now()
-	day1 := pickHeadAndTail(in, "2026-05-07", now, 20, 5)
-	day2 := pickHeadAndTail(in, "2026-05-08", now, 20, 5)
+	day1 := pickHeadAndTail(in, testUserID, "2026-05-07", now, 20, 5)
+	day2 := pickHeadAndTail(in, testUserID, "2026-05-08", now, 20, 5)
 	if len(day1) != 25 || len(day2) != 25 {
 		t.Fatalf("len mismatch: day1=%d day2=%d", len(day1), len(day2))
 	}
@@ -190,10 +201,11 @@ func TestPickHeadAndTail_TailFromBeyond2xHeadN(t *testing.T) {
 	in := make([]recommendation.Candidate, 0, 50)
 	for i := 0; i < 50; i++ {
 		// trackN = i+1, score descends: track 1 scores highest.
-		sim := float64(50-i) / 50.0
+		// Wide score gaps (1.0 per step) keep jitter from perturbing order.
+		sim := float64(50 - i)
 		in = append(in, makeCand(i+1, i+1, i+1, sim))
 	}
-	got := pickHeadAndTail(in, "2026-05-07", time.Now(), 5, 3)
+	got := pickHeadAndTail(in, testUserID, "2026-05-07", time.Now(), 5, 3)
 	if len(got) != 8 {
 		t.Fatalf("len = %d, want 8 (5 head + 3 tail)", len(got))
 	}
@@ -226,7 +238,7 @@ func TestPickTopN_DiversityCap(t *testing.T) {
 		makeCand(4, 13, 100, 0.7), // 4th by artist → dropped by cap
 		makeCand(5, 14, 100, 0.6), // 5th by artist → dropped by cap
 	}
-	got := pickTopN(in, "2026-05-07", time.Now(), 25)
+	got := pickTopN(in, testUserID, "2026-05-07", time.Now(), 25)
 	if len(got) != 3 {
 		t.Errorf("len = %d, want 3 (artist 100 capped at 3)", len(got))
 	}