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/v2/tests/recipient_info.cpp

@@ -0,0 +1,25 @@
+#include <gtest/gtest.h>
+#include <vanetza/common/byte_sequence.hpp>
+#include <vanetza/security/v2/recipient_info.hpp>
+#include <vanetza/security/v2/tests/check_recipient_info.hpp>
+#include <vanetza/security/tests/serialization.hpp>
+
+using namespace vanetza;
+using namespace vanetza::security;
+using namespace vanetza::security::v2;
+
+TEST(RecipientInfo, Serialize)
+{
+    EciesEncryptedKey ecies;
+    ecies.v = Compressed_Lsb_Y_0 { random_byte_sequence(field_size(PublicKeyAlgorithm::ECIES_NISTP256), 1337) };
+    auto cbuf = random_byte_sequence(field_size(SymmetricAlgorithm::AES128_CCM), 7331);
+    ecies.c = { cbuf.begin(), cbuf.end() };
+    auto tbuf = random_byte_sequence(ecies.t.size(), 1234);
+    std::copy_n(tbuf.begin(), ecies.t.size(), ecies.t.data());
+    RecipientInfo info;
+    info.enc_key = ecies;
+    auto certbuf = random_byte_sequence(info.cert_id.size(), 4321);
+    std::copy_n(certbuf.begin(), info.cert_id.size(), info.cert_id.data());
+
+    check(info, serialize_roundtrip(info, SymmetricAlgorithm::AES128_CCM));
+}