Add skeleton for the radio mesh simulator

test/mesh/l2_mesh.h

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+// Copyright (c) 2026 Cesanta Software Limited
+// All rights reserved
+//
+// Ethernet over radio, through a mesh. Mongoose sees an Ethernet interface,
+// its driver splits Ethernet frames into radio frames. Two parts plug in:
+// - struct mg_radio: a radio standard, e.g. mg_radio_802154
+// - struct mg_mesh: how the mesh is built and routed, e.g. mg_mesh_star
+
+#pragma once
+
+#include "mongoose.h"
+
+// A radio standard
+struct mg_radio {
+  const char *name;  // For logs
+};
+
+// A mesh strategy
+struct mg_mesh {
+  const char *name;  // For logs
+};