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/tsb_header.cpp

@@ -0,0 +1,26 @@
+#include "tsb_header.hpp"
+#include "serialization.hpp"
+
+namespace vanetza
+{
+namespace geonet
+{
+
+constexpr std::size_t TsbHeader::length_bytes;
+
+void serialize(const TsbHeader& hdr, OutputArchive& ar)
+{
+    serialize(hdr.sequence_number, ar);
+    serialize(hdr.reserved, ar);
+    serialize(hdr.source_position, ar);
+}
+
+void deserialize(TsbHeader& hdr, InputArchive& ar)
+{
+    deserialize(hdr.sequence_number, ar);
+    deserialize(hdr.reserved, ar);
+    deserialize(hdr.source_position, ar);
+}
+
+} // namespace geonet
+} // namespace vanetza