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/v3/certificate_cache.cpp

@@ -0,0 +1,59 @@
+#include <vanetza/security/v3/certificate_cache.hpp>
+#include <boost/optional/optional.hpp>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v3
+{
+
+const Certificate* CertificateCache::lookup(const HashedId8& digest) const
+{
+    auto found = m_storage.find(digest);
+    if (found != m_storage.end()) {
+        return &found->second;
+    } else {
+        return nullptr;
+    }
+}
+
+const Certificate* CertificateCache::lookup(const HashedId3& digest) const
+{
+    auto found = m_short_digests.find(digest);
+    if (found != m_short_digests.end()) {
+        return &found->second->second;
+    } else {
+        return nullptr;
+    }
+}
+
+void CertificateCache::store(Certificate cert)
+{
+    auto maybe_hash = cert.calculate_digest();
+    if (maybe_hash) {
+        CertificateMap::iterator it;
+        bool inserted;
+        std::tie(it, inserted) = m_storage.emplace(*maybe_hash, std::move(cert));
+        if (inserted) {
+            m_short_digests.emplace(truncate(*maybe_hash), it);
+            m_digests.insert(*maybe_hash);
+        }
+    }
+}
+
+bool CertificateCache::announce(const HashedId8& digest)
+{
+    bool inserted = false;
+    std::tie(std::ignore, inserted) = m_digests.insert(digest);
+    return inserted;
+}
+
+bool CertificateCache::is_known(const HashedId8& digest) const
+{
+    return m_digests.find(digest) != m_digests.end();
+}
+
+} // namespace v3
+} // namespace security
+} // namespace vanetza