feat(recommendation): add BuildSessionVector pure function

Pure aggregation per spec §6: Seed flag (true when prior < 3),
deduplicated Artists ordered by first appearance, Tags bag-of-counts
from tracks.genre (empty genres skipped), RecentTrackIDs preserving
input order. JSON round-trip verified.

internal/recommendation/sessionvector.go

@@ -0,0 +1,43 @@
+package recommendation
+
+import (
+	"github.com/jackc/pgx/v5/pgtype"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+type SessionVector struct {
+	Seed           bool           `json:"seed"`
+	Artists        []string       `json:"artists"`
+	Tags           map[string]int `json:"tags"`
+	RecentTrackIDs []string       `json:"recent_track_ids"`
+}
+
+func BuildSessionVector(priorTracks []dbq.Track) SessionVector {
+	v := SessionVector{
+		Seed:           len(priorTracks) < 3,
+		Artists:        []string{},
+		Tags:           map[string]int{},
+		RecentTrackIDs: []string{},
+	}
+	seen := map[string]bool{}
+	for _, t := range priorTracks {
+		artistID := uuidString(t.ArtistID)
+		if !seen[artistID] {
+			seen[artistID] = true
+			v.Artists = append(v.Artists, artistID)
+		}
+		if t.Genre != nil && *t.Genre != "" {
+			v.Tags[*t.Genre]++
+		}
+		v.RecentTrackIDs = append(v.RecentTrackIDs, uuidString(t.ID))
+	}
+	return v
+}
+
+func uuidString(u pgtype.UUID) string {
+	if !u.Valid {
+		return ""
+	}
+	return u.String()
+}