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/pqc/fndsa512.hpp

@@ -0,0 +1,62 @@
+#pragma once
+
+#include <vanetza/common/byte_buffer.hpp>
+#include <cstddef>
+#include <memory>
+
+namespace vanetza
+{
+namespace security
+{
+namespace pqc
+{
+
+constexpr std::size_t fndsa512_public_key_size = 897;
+constexpr std::size_t fndsa512_private_key_size = 1281;
+constexpr std::size_t fndsa512_signature_size = 666;
+
+struct PublicKey
+{
+    ByteBuffer bytes;
+};
+
+struct PrivateKey
+{
+    ByteBuffer bytes;
+};
+
+struct Signature
+{
+    ByteBuffer bytes;
+};
+
+struct KeyPair
+{
+    PublicKey public_key;
+    PrivateKey private_key;
+};
+
+/**
+ * Cryptographic operations for the experimental FN-DSA-512 profile.
+ *
+ * The interface is intentionally separate from security::Backend. Enabling
+ * the experimental profile therefore does not alter the established ECC
+ * backend contract or its key types.
+ */
+class Backend
+{
+public:
+    virtual KeyPair generate_key_pair() = 0;
+    virtual Signature sign(const PrivateKey&, const ByteBuffer& data) = 0;
+    virtual bool verify(const PublicKey&, const ByteBuffer& data, const Signature&) = 0;
+    virtual ~Backend() = default;
+};
+
+/**
+ * Create the liboqs-backed implementation used by the experimental profile.
+ */
+std::unique_ptr<Backend> create_fndsa512_backend();
+
+} // namespace pqc
+} // namespace security
+} // namespace vanetza