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/security/hashed_id.hpp

@@ -0,0 +1,44 @@
+#ifndef CE45A952_0EE7_4D20_82CB_D42BF87F5B15
+#define CE45A952_0EE7_4D20_82CB_D42BF87F5B15
+
+#include <vanetza/security/sha.hpp>
+#include <array>
+#include <cstdint>
+#include <functional>
+#include <string>
+
+namespace vanetza
+{
+namespace security
+{
+
+using HashedId8 = std::array<uint8_t, 8>;
+using HashedId3 = std::array<uint8_t, 3>;
+
+HashedId3 truncate(const HashedId8&);
+
+HashedId8 create_hashed_id8(const Sha256Digest&);
+HashedId8 create_hashed_id8(const Sha384Digest&);
+
+std::string to_string(const vanetza::security::HashedId3&);
+std::string to_string(const vanetza::security::HashedId8&);
+
+} // namespace security
+} // namespace vanetza
+
+namespace std
+{
+// std::hash specialization for HashedId3
+template<> struct hash<vanetza::security::HashedId3>
+{
+    size_t operator()(const vanetza::security::HashedId3&) const;
+};
+
+/// std::hash specialization for HashedId8
+template<> struct hash<vanetza::security::HashedId8>
+{
+    size_t operator()(const vanetza::security::HashedId8&) const;
+};
+} // namespace std
+
+#endif /* CE45A952_0EE7_4D20_82CB_D42BF87F5B15 */