feat(multiplayer): add Steam transport foundation

Game/scripts/net_transport.gd

@@ -0,0 +1,26 @@
+class_name NetTransport
+extends RefCounted
+
+# Narrow construction boundary for Godot's MultiplayerPeer implementations.
+# NetworkManager owns polling, RPC policy, and lifecycle; a transport only
+# creates a peer.  Keeping that split means ENet remains a first-class path
+# while Steam can use SDR without duplicating the rest of the networking code.
+
+func transport_id() -> String:
+	return ""
+
+
+func is_available() -> bool:
+	return false
+
+
+func unavailable_reason() -> String:
+	return "transport is unavailable"
+
+
+func create_server(_port: int, _max_clients: int) -> Dictionary:
+	return {"error": ERR_UNAVAILABLE, "peer": null, "reason": unavailable_reason()}
+
+
+func create_client(_address: String, _port: int) -> Dictionary:
+	return {"error": ERR_UNAVAILABLE, "peer": null, "reason": unavailable_reason()}