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.cpp

@@ -0,0 +1,36 @@
+#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>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+void serialize(const ConstAccessiblePdu& pdu, OutputArchive& ar)
+{
+    serialize(pdu.basic(), ar);
+    if (pdu.secured()) {
+        security::serialize(ar, *pdu.secured());
+    } else {
+        geonet::serialize(pdu.common(), ar);
+        boost::serialize(pdu.extended_variant(), ar);
+    }
+}
+
+std::size_t get_length(const ConstAccessiblePdu& pdu)
+{
+    std::size_t length = BasicHeader::length_bytes;
+    if (pdu.secured()) {
+        length += security::get_size(*pdu.secured());
+    } else {
+        length += CommonHeader::length_bytes;
+        length += get_length(pdu.extended_variant());
+    }
+    return length;
+}
+
+} // namespace geonet
+} // namespace vanetza