feat: add player-scoped assignment recovery

Game/scripts/assignment_state.gd

@@ -0,0 +1,45 @@
+class_name AssignmentState
+extends RefCounted
+
+# Verified assignment-ready manifest returned by the control plane. The join
+# authorisation is retained in memory only and is never written to the restart
+# snapshot; transport installation belongs to the explicit ENet/Steam layer.
+
+var available := false
+var match_id := ""
+var server_id := ""
+var slot := -1
+var expires_at := ""
+var protocol_version := 0
+var transport := ""
+var join_authorisation := ""
+var error_message := ""
+
+
+func apply(payload: Dictionary) -> bool:
+	for key in ["match_id", "server_id", "player_id", "slot", "expires_at", "protocol_version", "transport", "join_authorisation"]:
+		if not payload.has(key):
+			return _reject("Assignment response is missing " + key)
+	if not payload["match_id"] is String or not payload["server_id"] is String or not payload["player_id"] is String or not payload["slot"] is int or not payload["expires_at"] is String or not payload["protocol_version"] is int or not payload["transport"] is String or not payload["join_authorisation"] is String:
+		return _reject("Assignment response contains invalid types")
+	var next_match_id := String(payload["match_id"])
+	var next_server_id := String(payload["server_id"])
+	var next_transport := String(payload["transport"])
+	if next_match_id.is_empty() or next_server_id.is_empty() or String(payload["player_id"]).is_empty() or int(payload["slot"]) < 0 or int(payload["slot"]) > 5 or int(payload["protocol_version"]) < 1 or (next_transport != "enet" and next_transport != "steam_sdr") or String(payload["expires_at"]).is_empty() or String(payload["join_authorisation"]).is_empty():
+		return _reject("Assignment response contains invalid values")
+	match_id = next_match_id
+	server_id = next_server_id
+	slot = int(payload["slot"])
+	expires_at = String(payload["expires_at"])
+	protocol_version = int(payload["protocol_version"])
+	transport = next_transport
+	join_authorisation = String(payload["join_authorisation"])
+	available = true
+	error_message = ""
+	return true
+
+
+func _reject(reason: String) -> bool:
+	available = false
+	error_message = reason
+	return false