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.hpp

@@ -0,0 +1,43 @@
+#ifndef PDU_CONVERSION_HPP_XLCSI42E
+#define PDU_CONVERSION_HPP_XLCSI42E
+
+#include <vanetza/common/byte_buffer.hpp>
+#include <vanetza/common/byte_buffer_convertible.hpp>
+#include <vanetza/geonet/pdu.hpp>
+#include <memory>
+
+namespace vanetza
+{
+namespace convertible
+{
+
+template<>
+struct byte_buffer_impl<std::unique_ptr<vanetza::geonet::Pdu>> : public byte_buffer
+{
+    byte_buffer_impl(std::unique_ptr<vanetza::geonet::Pdu> pdu) :
+        m_pdu(std::move(pdu)) {}
+
+    void convert(ByteBuffer& dest) const override;
+    std::size_t size() const override;
+    std::unique_ptr<byte_buffer> duplicate() const override;
+
+    const std::unique_ptr<vanetza::geonet::Pdu> m_pdu;
+};
+
+} // namespace convertible
+
+namespace geonet
+{
+
+/**
+ * Fetch PDU from byte buffer convertible
+ * \param conv source convertible
+ * \return PDU pointer or nullptr if cast failed
+ * */
+Pdu* pdu_cast(ByteBufferConvertible& conv);
+
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* PDU_CONVERSION_HPP_XLCSI42E */
+