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/guc_header.hpp

@@ -0,0 +1,29 @@
+#ifndef GUC_HEADER_HPP_UEGE3IFW
+#define GUC_HEADER_HPP_UEGE3IFW
+
+#include <vanetza/geonet/position_vector.hpp>
+#include <vanetza/geonet/sequence_number.hpp>
+#include <cstddef>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+struct GeoUnicastHeader
+{
+    static constexpr std::size_t length_bytes = 4 +
+        LongPositionVector::length_bytes +
+        ShortPositionVector::length_bytes;
+
+    SequenceNumber sequence_number;
+    uint16_t reserved;
+    LongPositionVector source_position;
+    ShortPositionVector destination_position;
+};
+
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* GUC_HEADER_HPP_UEGE3IFW */
+