feat(recommendation): add ContextualMatchScore (max over non-seed likes)

internal/recommendation/similarity.go

@@ -74,3 +74,28 @@ func setJaccardSlice(a, b []string) float64 {
 	}
 	return float64(intersect) / float64(union)
 }
+
+// ContextualMatchScore returns the maximum Similarity between the current
+// session vector and any non-seed entry in likes. Returns 0 when:
+//   - current.Seed is true (no meaningful current context)
+//   - likes is empty after filtering out Seed=true entries
+//
+// The "max" semantics means a single strong contextual match dominates
+// over many weak ones — we want to surface the track because it was liked
+// in *some* matching context, not because it was vaguely-liked in many.
+func ContextualMatchScore(current SessionVector, likes []SessionVector, w SimilarityWeights) float64 {
+	if current.Seed {
+		return 0.0
+	}
+	best := 0.0
+	for _, l := range likes {
+		if l.Seed {
+			continue
+		}
+		s := Similarity(current, l, w)
+		if s > best {
+			best = s
+		}
+	}
+	return best
+}