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/common/byte_buffer_sink.cpp

@@ -0,0 +1,19 @@
+#include "byte_buffer_sink.hpp"
+#include <algorithm>
+#include <iterator>
+
+namespace vanetza
+{
+
+byte_buffer_sink::byte_buffer_sink(ByteBuffer& buffer) : m_buffer(buffer)
+{
+}
+
+std::streamsize byte_buffer_sink::write(const char_type* s, std::streamsize n)
+{
+    std::copy(s, s + n, std::back_inserter(m_buffer));
+    return n;
+}
+
+} // namespace vanetza
+