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 {