First implementation of graph vehicles

vehicle/graph/graph_preview.h

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+/*
+This Source Code Form is subject to the
+terms of the Mozilla Public License, v.
+2.0. If a copy of the MPL was not
+distributed with this file, You can
+obtain one at
+http://mozilla.org/MPL/2.0/.
+*/
+
+#pragma once
+
+// live preview of the vehicle graphs for the external editor: a TCP server on the local machine, speaking json lines
+// (protocol 1, see tools/vehicle_graph/README.md). off unless enabled with -graph-preview [port] or with
+// "vehiclegraph_preview yes [port]" in eu07.ini. everything runs on the simulation thread: the connections are served
+// by polling, once per frame
+namespace vgraph
+{
+
+// serves the preview connections; does nothing when the preview is disabled
+void preview_update();
+
+} // namespace vgraph