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/geodesy/geodesy.hpp

@@ -0,0 +1,32 @@
+#pragma once
+
+#include <vanetza/geodesy/position.hpp>
+
+namespace vanetza
+{
+namespace geodesy
+{
+
+/**
+ * Get distance between two geodetic positions.
+ * Delegates to the best available backend (GeographicLib if available, else haversine).
+ * \param a first position
+ * \param b second position
+ * \return distance in meters (always positive) or NaN for invalid input
+ */
+units::Length distance(const GeodeticPosition& a, const GeodeticPosition& b);
+
+/**
+ * Derive cartesian position ENU from geodetic WGS84 coordinates
+ * and a WGS84 reference point which becomes the cartesian origin.
+ * Delegates to the best available backend.
+ * \param origin WGS84 reference point becoming origin
+ * \param position Calculate cartesian coordinates for this point
+ * \return Cartesian coordinates of position relative to origin
+ */
+CartesianPosition local_cartesian(
+        const GeodeticPosition& origin,
+        const GeodeticPosition& position);
+
+} // namespace geodesy
+} // namespace vanetza