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/trust_store.cpp

@@ -0,0 +1,35 @@
+#include <vanetza/security/v2/trust_store.hpp>
+#include <stdexcept>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+void TrustStore::insert(const Certificate& certificate)
+{
+    if (certificate.subject_info.subject_type != SubjectType::Root_CA) {
+        throw std::runtime_error("Only root certificate authorities may be added to the trust store");
+    }
+
+    HashedId8 id = calculate_hash(certificate);
+    m_certificates.insert(std::make_pair(id, certificate));
+}
+
+std::list<Certificate> TrustStore::lookup(HashedId8 id) const
+{
+    using iterator = std::multimap<HashedId8, Certificate>::const_iterator;
+    std::pair<iterator, iterator> range = m_certificates.equal_range(id);
+
+    std::list<Certificate> matches;
+    for (auto item = range.first; item != range.second; ++item) {
+        matches.push_back(item->second);
+    }
+    return matches;
+}
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza