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.cpp

@@ -0,0 +1,39 @@
+#include <vanetza/geonet/cbf_packet_identifier.hpp>
+#include <vanetza/geonet/cbf_packet_buffer.hpp>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+CbfPacketIdentifier identifier(const CbfPacket& packet)
+{
+    return identifier(packet.source(), packet.sequence_number());
+}
+
+CbfPacketIdentifier identifier(const Address& source, SequenceNumber sn)
+{
+    return std::make_tuple(source, sn);
+}
+
+} // namespace geonet
+} // namespace vanetza
+
+namespace std
+{
+
+size_t hash<vanetza::geonet::CbfPacketIdentifier>::operator()(const vanetza::geonet::CbfPacketIdentifier& id) const
+{
+    using vanetza::geonet::Address;
+    using vanetza::geonet::SequenceNumber;
+    static_assert(tuple_size<vanetza::geonet::CbfPacketIdentifier>::value == 2, "Unexpected identifier tuple");
+
+    std::size_t seed = 0;
+    const Address& source = get<0>(id);
+    boost::hash_combine(seed, std::hash<Address>()(source));
+    const SequenceNumber& sn = get<1>(id);
+    boost::hash_combine(seed, static_cast<SequenceNumber::value_type>(sn));
+    return seed;
+}
+
+} // namespace std