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.hpp

@@ -0,0 +1,58 @@
+#ifndef PDU_HPP_PQEC9PDO
+#define PDU_HPP_PQEC9PDO
+
+#include <vanetza/geonet/header_variant.hpp>
+#include <vanetza/geonet/serialization.hpp>
+#include <vanetza/security/secured_message.hpp>
+#include <cstddef>
+#include <memory>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+struct BasicHeader;
+struct CommonHeader;
+
+class Pdu;
+
+class ConstAccessiblePdu
+{
+public:
+    using SecuredMessage = security::SecuredMessage;
+
+    virtual const BasicHeader& basic() const = 0;
+    virtual const CommonHeader& common() const = 0;
+    virtual const SecuredMessage* secured() const = 0;
+    virtual HeaderConstRefVariant extended_variant() const = 0;
+    virtual std::unique_ptr<Pdu> clone() const = 0;
+    virtual ~ConstAccessiblePdu() = default;
+};
+
+class Pdu : public ConstAccessiblePdu
+{
+public:
+    using ConstAccessiblePdu::basic;
+    using ConstAccessiblePdu::common;
+    using ConstAccessiblePdu::secured;
+
+    virtual BasicHeader& basic() = 0;
+    virtual CommonHeader& common() = 0;
+    virtual SecuredMessage* secured() = 0;
+    virtual void secured(const SecuredMessage&) = 0;
+};
+
+void serialize(const ConstAccessiblePdu&, OutputArchive&);
+inline void serialize(const Pdu& pdu, OutputArchive& ar)
+{
+    serialize(static_cast<const ConstAccessiblePdu&>(pdu), ar);
+}
+
+std::size_t get_length(const ConstAccessiblePdu&);
+
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* PDU_HPP_PQEC9PDO */
+