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/pdu_conversion.cpp

@@ -0,0 +1,49 @@
+#include "pdu_conversion.hpp"
+#include "serialization_buffer.hpp"
+#include <cassert>
+
+namespace vanetza
+{
+namespace convertible
+{
+
+typedef std::unique_ptr<vanetza::geonet::Pdu> PduPtr;
+
+void byte_buffer_impl<PduPtr>::convert(ByteBuffer& dest) const
+{
+    assert(m_pdu);
+    dest.clear();
+    geonet::serialize_into_buffer(*m_pdu, dest);
+}
+
+std::size_t byte_buffer_impl<PduPtr>::size() const
+{
+    assert(m_pdu);
+    return get_length(*m_pdu);
+}
+
+std::unique_ptr<byte_buffer> byte_buffer_impl<PduPtr>::duplicate() const
+{
+    assert(m_pdu);
+    PduPtr duplicate { m_pdu->clone() };
+    std::unique_ptr<byte_buffer> result {
+        new byte_buffer_impl<PduPtr>(std::move(duplicate))
+    };
+    return result;
+}
+
+} // namespace convertible
+
+namespace geonet
+{
+
+Pdu* pdu_cast(ByteBufferConvertible& convertible)
+{
+    using convertible_pdu_t = convertible::byte_buffer_impl<std::unique_ptr<Pdu>>;
+    auto convertible_pdu = dynamic_cast<convertible_pdu_t*>(convertible.ptr());
+    return convertible_pdu ? convertible_pdu->m_pdu.get() : nullptr;
+}
+
+} // namespace geonet
+} // namespace vanetza
+