Keep the colleague's microbu-esp32c5 tree in this repository

obu-firmware builds against vanetza-idf from microbu-esp32c5/external, but
that tree was gitignored, so a clone of this repository could not build the
firmware it ships. It is now committed here as ordinary files in its own
folder, microbu-esp32c5/: the colleague's commit cf4b99f plus the V2X2MAP
bridge's signature verification (--trust) used on the bench. Nothing is
fetched from or pushed to the colleague's repository; this repository and
its remotes carry everything. The folder's own .gitignore keeps build output,
downloaded components and private key material out, as it did there; the
committed file set is identical to that repository's tracked files.

The ESP32-C5 is still flashed from obu-firmware/, which only takes
vanetza-idf from microbu-esp32c5/, so the two stay separate folders.
FLASHING.md says how to take a newer version of the colleague's tree (copy
it over the folder, rebuild, test, commit).

microbu-esp32c5/external/vanetza-idf/vanetza/net/socket_traits.hpp

@@ -0,0 +1,48 @@
+#ifndef SOCKET_TRAITS_HPP_
+#define SOCKET_TRAITS_HPP_
+
+#include <vanetza/net/osi_layer.hpp>
+#include <type_traits>
+
+namespace vanetza
+{
+
+template<typename SOCKET>
+struct socket_layers
+{
+    static const OsiLayer min = SOCKET::min_layer;
+    static const OsiLayer max = SOCKET::max_layer;
+
+    static_assert(min <= max, "Corrupt layer ordering");
+};
+
+namespace pdu_tags
+{
+
+struct pdu_tag {};
+struct empty_pdu_tag : public pdu_tag {};
+
+struct phy_tag : public pdu_tag {};
+struct mk2_tag : public phy_tag {};
+
+struct mac_tag : public pdu_tag {};
+struct ethernet_tag : public mac_tag {};
+
+} // namespace pdu_tags
+
+template<typename SOCKET, OsiLayer LAYER>
+struct socket_layer_pdu
+{
+    typedef pdu_tags::empty_pdu_tag tag;
+};
+
+template<OsiLayer LAYER, typename TAG, class SOCKET>
+bool constexpr pdu_match(const SOCKET&)
+{
+    return std::is_same<TAG, typename socket_layer_pdu<SOCKET, LAYER>::tag>::value;
+}
+
+} // namespace vanetza
+
+#endif // SOCKET_TRAITS_HPP_
+