fix(multiplayer): enforce client state transitions

Game/scripts/matchmaking_state.gd

@@ -74,6 +74,8 @@ func apply_ticket_update(update: Dictionary) -> bool:
 	var incoming_state := String(update["state"])
 	if not _is_ticket_state(incoming_state):
 		return _request_resync(self.ticket_id)
+	if not _is_legal_ticket_transition(phase, incoming_state):
+		return _request_resync(self.ticket_id)
 	revision = incoming_revision
 	phase = incoming_state
 	if update.has("playlist"):
@@ -248,5 +250,23 @@ func _is_ticket_state(value: String) -> bool:
 	return value in [QUEUED, PROPOSED, ACCEPTED, ALLOCATING, PROCESS_READY, ASSIGNMENT_READY, ASSIGNED, CONNECTING, LIVE, RESULT_PENDING, COMPLETED, CANCELLED, EXPIRED, FAILED]
 
 
+func _is_legal_ticket_transition(from: String, to: String) -> bool:
+	if from == to:
+		return true
+	var transitions := {
+		QUEUED: [PROPOSED, CANCELLED, EXPIRED],
+		PROPOSED: [QUEUED, ACCEPTED, CANCELLED, EXPIRED],
+		ACCEPTED: [QUEUED, ALLOCATING, CANCELLED, FAILED],
+		ALLOCATING: [PROCESS_READY, FAILED, CANCELLED],
+		PROCESS_READY: [ASSIGNMENT_READY, FAILED, CANCELLED],
+		ASSIGNMENT_READY: [ASSIGNED, FAILED, CANCELLED],
+		ASSIGNED: [CONNECTING, FAILED, CANCELLED],
+		CONNECTING: [LIVE, FAILED, EXPIRED],
+		LIVE: [RESULT_PENDING, FAILED],
+		RESULT_PENDING: [COMPLETED, FAILED],
+	}
+	return transitions.has(from) and to in transitions[from]
+
+
 func _has_string(value: Dictionary, key: String) -> bool:
 	return value.has(key) and value[key] is String and not String(value[key]).is_empty()