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/issuer_lookup.hpp

@@ -0,0 +1,31 @@
+#pragma once
+#include <vanetza/security/hashed_id.hpp>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v3
+{
+
+class Certificate;
+
+/**
+ * Resolves a signing certificate's issuer by digest, typically backed by
+ * PKI-managed trust material rather than the peer-AT CertificateCache.
+ */
+class IssuerLookup
+{
+public:
+    virtual ~IssuerLookup() = default;
+
+    /**
+     * \param digest issuer's HashedId8
+     * \return pointer to the issuer certificate, or nullptr if unknown.
+     */
+    virtual const Certificate* find_issuer(const HashedId8& digest) const = 0;
+};
+
+} // namespace v3
+} // namespace security
+} // namespace vanetza