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/tests/serialization.hpp

@@ -0,0 +1,49 @@
+#ifndef SERIALIZATION_HPP_ZFDJQSWI
+#define SERIALIZATION_HPP_ZFDJQSWI
+
+#include <vanetza/common/archives.hpp>
+#include <vanetza/common/serialization.hpp>
+#include <sstream>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+/**
+ * \brief Serialize and deserialize an object
+ *
+ * Source object is serialized and deserialized
+ * object form this binary representation is returned.
+ *
+ * \tparam T the type of the result
+ * \param source serialize from this object
+ * \return deserialized object (should be equal to source)
+ */
+template<typename T>
+T serialize_roundtrip(const T& source)
+{
+    std::stringstream stream;
+    OutputArchive oa(stream);
+    serialize(source, oa);
+
+    T result;
+    InputArchive ia(stream);
+    deserialize(result, ia);
+
+    return result;
+}
+
+template<typename T>
+std::size_t serialize_length(const T& source)
+{
+    std::stringstream stream;
+    OutputArchive oa(stream);
+    serialize(source, oa);
+    return stream.tellp();
+}
+
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* SERIALIZATION_HPP_ZFDJQSWI */