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/net/packet.cpp

@@ -0,0 +1,38 @@
+#include <vanetza/net/packet.hpp>
+
+namespace vanetza
+{
+
+std::unique_ptr<DownPacket> duplicate(const PacketVariant& packet)
+{
+    struct duplication_visitor : public boost::static_visitor<>
+    {
+        void operator()(const CohesivePacket& packet)
+        {
+            m_duplicate.reset(new ChunkPacket());
+            for (auto layer : osi_layers) {
+                const auto source = packet[layer];
+                auto& dest = m_duplicate->layer(layer);
+                dest = ByteBuffer(source.begin(), source.end());
+            }
+        }
+
+        void operator()(const ChunkPacket& packet)
+        {
+            m_duplicate = duplicate(packet);
+        }
+
+        std::unique_ptr<ChunkPacket> m_duplicate;
+    };
+
+    duplication_visitor visitor;
+    boost::apply_visitor(visitor, packet);
+    return std::move(visitor.m_duplicate);
+}
+
+std::unique_ptr<DownPacket> duplicate(const DownPacket& packet)
+{
+    return std::unique_ptr<DownPacket> { new DownPacket(packet) };
+}
+
+} // namespace vanetza