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/variant_pdu.hpp

@@ -0,0 +1,50 @@
+#ifndef VARIANT_PDU_HPP_PHYRXMBW
+#define VARIANT_PDU_HPP_PHYRXMBW
+
+#include <vanetza/geonet/basic_header.hpp>
+#include <vanetza/geonet/common_header.hpp>
+#include <vanetza/geonet/header_variant.hpp>
+#include <vanetza/geonet/pdu.hpp>
+#include <vanetza/security/secured_message.hpp>
+#include <boost/optional/optional.hpp>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+class VariantPdu : public Pdu
+{
+public:
+    using SecuredMessage = security::SecuredMessage;
+
+    VariantPdu() = default;
+    VariantPdu(const VariantPdu&);
+    VariantPdu& operator=(const VariantPdu&);
+    VariantPdu(const Pdu&);
+    VariantPdu& operator=(const Pdu&);
+
+    BasicHeader& basic() override;
+    const BasicHeader& basic() const override;
+    CommonHeader& common() override;
+    const CommonHeader& common() const override;
+    HeaderVariant& extended_variant();
+    HeaderConstRefVariant extended_variant() const override;
+    SecuredMessage* secured() override;
+    const SecuredMessage* secured() const override;
+    void secured(const SecuredMessage&) override;
+    void secured(SecuredMessage&&);
+    std::unique_ptr<Pdu> clone() const override;
+
+private:
+    BasicHeader m_basic;
+    CommonHeader m_common;
+    HeaderVariant m_extended;
+    boost::optional<SecuredMessage> m_secured;
+};
+
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* VARIANT_PDU_HPP_PHYRXMBW */
+