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/io_vector.cpp

@@ -0,0 +1,39 @@
+#include "io_vector.hpp"
+#include "buffer_packet.hpp"
+
+namespace vanetza
+{
+
+void IoVector::clear()
+{
+    m_vector.clear();
+}
+
+void IoVector::append(const void* base, std::size_t length)
+{
+    iovec node;
+    node.iov_base = const_cast<void*>(base);
+    node.iov_len = length;
+    m_vector.push_back(node);
+}
+
+void IoVector::append(const BufferPacket& packet)
+{
+    for (auto& kv : packet) {
+        const ByteBuffer& buffer = kv.second;
+        append(buffer.data(), buffer.size());
+    }
+}
+
+std::size_t IoVector::length() const
+{
+    return m_vector.size();
+}
+
+const iovec* IoVector::base() const
+{
+    return m_vector.empty() ? nullptr : &m_vector[0];
+}
+
+} // namespace vanetza
+