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/buffer_packet.cpp

@@ -0,0 +1,44 @@
+#include "buffer_packet.hpp"
+
+namespace vanetza
+{
+
+BufferPacket::BufferPacket()
+{
+}
+
+void BufferPacket::swap(OsiLayer layer, ByteBuffer& replacement)
+{
+    ByteBuffer& stored = mBuffers[layer];
+    stored.swap(replacement);
+}
+
+const ByteBuffer& BufferPacket::operator[](OsiLayer layer) const
+{
+    auto match = mBuffers.find(layer);
+    if (match == mBuffers.end()) {
+        static const ByteBuffer scEmptyBuffer;
+        return scEmptyBuffer;
+    } else {
+        return match->second;
+    }
+}
+
+std::size_t BufferPacket::size() const
+{
+    std::size_t packet_size = 0;
+    for (const auto& it : *this) {
+        packet_size += it.second.size();
+    }
+    return packet_size;
+}
+
+void BufferPacket::clear()
+{
+    for (auto& it : mBuffers) {
+        it.second.clear();
+    }
+}
+
+} // namespace vanetza
+