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/subject_info.cpp

@@ -0,0 +1,23 @@
+#include <gtest/gtest.h>
+#include <vanetza/common/byte_sequence.hpp>
+#include <vanetza/security/v2/subject_info.hpp>
+#include <vanetza/security/v2/tests/check_subject_info.hpp>
+#include <vanetza/security/tests/serialization.hpp>
+
+using namespace vanetza::security::v2;
+
+TEST(SubjectInfo, Serialization)
+{
+    SubjectInfo sub;
+    sub.subject_type = SubjectType::Root_CA;
+    sub.subject_name = vanetza::random_byte_sequence(40);
+    check(sub, serialize_roundtrip(sub));
+}
+
+TEST(SubjectInfo, WebValidator_Size)
+{
+    SubjectInfo info;
+    info.subject_type = SubjectType::Authorization_Ticket;
+
+    EXPECT_EQ(2, get_size(info));
+}