feat(recommendation): add Shuffle orchestrator

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).

internal/recommendation/shuffle.go

@@ -0,0 +1,48 @@
+package recommendation
+
+import (
+	"sort"
+	"time"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+// Candidate pairs a track with the inputs needed to score it.
+type Candidate struct {
+	Track  dbq.Track
+	Inputs ScoringInputs
+}
+
+// Shuffle scores each candidate, sorts descending by score, and returns
+// the top `limit` candidates. limit <= 0 returns nil; nil input returns
+// nil. Pure — no IO, no global state beyond the rng callback.
+func Shuffle(
+	candidates []Candidate,
+	weights ScoringWeights,
+	now time.Time,
+	rng func() float64,
+	limit int,
+) []Candidate {
+	if len(candidates) == 0 || limit <= 0 {
+		return nil
+	}
+	scored := make([]struct {
+		c     Candidate
+		score float64
+	}, len(candidates))
+	for i, c := range candidates {
+		scored[i].c = c
+		scored[i].score = Score(c.Inputs, weights, now, rng)
+	}
+	sort.Slice(scored, func(i, j int) bool {
+		return scored[i].score > scored[j].score
+	})
+	if limit > len(scored) {
+		limit = len(scored)
+	}
+	out := make([]Candidate, limit)
+	for i := 0; i < limit; i++ {
+		out[i] = scored[i].c
+	}
+	return out
+}