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/shb_header.hpp

@@ -0,0 +1,30 @@
+#ifndef SHB_HEADER_HPP_MRLDRPNK
+#define SHB_HEADER_HPP_MRLDRPNK
+
+#include <vanetza/geonet/dcc_field.hpp>
+#include <vanetza/geonet/position_vector.hpp>
+#include <cstdint>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+struct ShbHeader
+{
+    ShbHeader();
+
+    static constexpr std::size_t length_bytes = 4 + LongPositionVector::length_bytes;
+
+    LongPositionVector source_position;
+    DccField dcc; /* < this field is "reserved" in 102 636-4-1 */
+};
+
+void serialize(const ShbHeader&, OutputArchive&);
+void deserialize(ShbHeader&, InputArchive&);
+
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* SHB_HEADER_HPP_MRLDRPNK */
+