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/vanetza/geonet/cbf_packet_identifier.hpp

@@ -0,0 +1,33 @@
+#ifndef CBF_PACKET_IDENTIFIER_HPP_HC6PLCML
+#define CBF_PACKET_IDENTIFIER_HPP_HC6PLCML
+
+#include <vanetza/geonet/address.hpp>
+#include <vanetza/geonet/sequence_number.hpp>
+#include <functional>
+#include <tuple>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+class CbfPacket;
+
+using CbfPacketIdentifier = std::tuple<Address, SequenceNumber>;
+CbfPacketIdentifier identifier(const CbfPacket&);
+CbfPacketIdentifier identifier(const Address&, SequenceNumber);
+
+} // namespace geonet
+} // namespace vanetza
+
+namespace std
+{
+/// std::hash specialization for CbfPacketIdentifier
+template<> struct hash<vanetza::geonet::CbfPacketIdentifier>
+{
+    size_t operator()(const vanetza::geonet::CbfPacketIdentifier&) const;
+};
+} // namespace std
+
+#endif /* CBF_PACKET_IDENTIFIER_HPP_HC6PLCML */
+