feat: validate queue compatibility metadata

server/domain/matcher.go

@@ -15,14 +15,26 @@ const (
 	RatingWidenPeriod  = 30.0
 )
 
+// QueueSpec is the compatibility contract selected by the authenticated
+// client. CandidateProviderV2 may use it to resolve a server-owned projection
+// from the verified account and current deployment configuration.
+type QueueSpec struct {
+	Playlist        Playlist
+	ClientBuild     string
+	ProtocolVersion int
+}
+
 // Candidate is the server-side projection of a verified, live queue ticket.
 // RTT values come from backend probes, never from the client request body.
 type Candidate struct {
-	TicketID     string
-	PlayerID     string
-	Rating       float64
-	EnqueuedAt   time.Time
-	PredictedRTT map[string]float64
+	TicketID        string
+	PlayerID        string
+	Playlist        Playlist
+	ClientBuild     string
+	ProtocolVersion int
+	Rating          float64
+	EnqueuedAt      time.Time
+	PredictedRTT    map[string]float64
 }
 
 type Selection struct {

server/domain/queue.go

@@ -26,6 +26,7 @@ type QueueTicket struct {
 	TicketID   string
 	PlayerID   string
 	Candidate  Candidate
+	Playlist   Playlist
 	State      State
 	Revision   uint64
 	EnqueuedAt time.Time
@@ -70,7 +71,7 @@ func (q *Queue) Create(playerID, ticketID, idempotencyKey string, candidate Cand
 	if _, ok := q.tickets[ticketID]; ok {
 		return QueueTicket{}, fmt.Errorf("%w: ticket ID already exists", ErrConflict)
 	}
-	ticket := QueueTicket{TicketID: ticketID, PlayerID: playerID, Candidate: candidate, State: Queued, EnqueuedAt: now, ExpiresAt: now.Add(QueueExpiryWindow)}
+	ticket := QueueTicket{TicketID: ticketID, PlayerID: playerID, Candidate: candidate, Playlist: candidate.Playlist, State: Queued, EnqueuedAt: now, ExpiresAt: now.Add(QueueExpiryWindow)}
 	q.tickets[ticketID] = ticket
 	q.byPlayer[playerID] = ticketID
 	q.mutations[idempotencyKey] = queueMutation{digest: digest, ticket: ticket}