Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/external/vanetza-idf/tools/socktap/udp_link.hpp

@@ -0,0 +1,35 @@
+#ifndef UDP_LINK_HPP_A16QFBX3
+#define UDP_LINK_HPP_A16QFBX3
+
+#include "link_layer.hpp"
+#include <vanetza/common/byte_buffer.hpp>
+#include <boost/asio/ip/udp.hpp>
+#include <array>
+
+// forward declaration
+class EthernetDevice;
+
+class UdpLink : public LinkLayer
+{
+public:
+    UdpLink(boost::asio::io_context&, const boost::asio::ip::udp::endpoint&, const EthernetDevice&);
+
+    void indicate(IndicationCallback) override;
+    void request(const vanetza::access::DataRequest&, std::unique_ptr<vanetza::ChunkPacket>) override;
+
+private:
+    void do_receive();
+
+    static constexpr std::size_t layers_ = num_osi_layers(vanetza::OsiLayer::Link, vanetza::OsiLayer::Application);
+
+    boost::asio::ip::udp::endpoint multicast_endpoint_;
+    boost::asio::ip::udp::socket tx_socket_;
+    boost::asio::ip::udp::socket rx_socket_;
+    std::array<vanetza::ByteBuffer, layers_> tx_buffers_;
+    vanetza::ByteBuffer rx_buffer_;
+    boost::asio::ip::udp::endpoint rx_endpoint_;
+    IndicationCallback callback_;
+};
+
+#endif /* UDP_LINK_HPP_A16QFBX3 */
+