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/secured_pdu.hpp

@@ -0,0 +1,63 @@
+#ifndef SECURED_PDU_HPP_TVERYI91
+#define SECURED_PDU_HPP_TVERYI91
+
+#include <vanetza/common/byte_buffer_convertible.hpp>
+#include <vanetza/geonet/common_header.hpp>
+#include <vanetza/geonet/extended_pdu.hpp>
+#include <vanetza/geonet/header_variant.hpp>
+#include <vanetza/geonet/serialization.hpp>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+/**
+ * Secured PDU consists of GeoNetworking headers belonging to a secured message's payload
+ */
+class SecuredPdu
+{
+public:
+    template<typename EXT>
+    SecuredPdu(const ExtendedPdu<EXT>& pdu) :
+        common(pdu.common()), extended(pdu.extended())
+    {
+    }
+
+    SecuredPdu(const Pdu&);
+
+    CommonHeader common;
+    HeaderVariant extended;
+};
+
+void serialize(const SecuredPdu&, OutputArchive&);
+
+/**
+ * Cast secured PDU from byte buffer convertible
+ * \param conv source convertible
+ * \return secured PDU pointer or nullptr if cast failed
+ */
+SecuredPdu* secured_pdu_cast(ByteBufferConvertible&);
+
+} // namespace geonet
+
+namespace convertible
+{
+
+template<>
+struct byte_buffer_impl<geonet::SecuredPdu> : public byte_buffer
+{
+    byte_buffer_impl(const geonet::SecuredPdu&);
+    void convert(ByteBuffer&) const override;
+    std::size_t size() const override;
+    std::unique_ptr<byte_buffer> duplicate() const override;
+
+    geonet::SecuredPdu pdu;
+};
+
+} // namespace convertible
+
+} // namespace vanetza
+
+#endif /* SECURED_PDU_HPP_TVERYI91 */
+