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_sequence.cpp

@@ -0,0 +1,22 @@
+#include <vanetza/common/byte_sequence.hpp>
+#include <algorithm>
+#include <random>
+
+namespace vanetza
+{
+
+ByteBuffer random_byte_sequence(std::size_t length, int seed)
+{
+    ByteBuffer buffer(length);
+    std::generate(buffer.begin(), buffer.end(), random_byte_generator(seed));
+    return buffer;
+}
+
+std::function<uint8_t()> random_byte_generator(int seed)
+{
+    std::mt19937 rng;
+    rng.seed(seed);
+    return [rng]() mutable { return rng(); };
+}
+
+} // namespace vanetza