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/access/data_rates.cpp

@@ -0,0 +1,25 @@
+#include "data_rates.hpp"
+#include <cassert>
+
+namespace vanetza
+{
+namespace access
+{
+
+std::size_t DataRateG5::data_length(std::size_t psdu) const
+{
+    static const unsigned service_bits = 16;
+    static const unsigned tail_bits = 6;
+
+    const unsigned body = service_bits + tail_bits + psdu * 8;
+    unsigned padding = body % m_coded_bits_per_symbol;
+    if (padding > 0) {
+        padding = m_coded_bits_per_symbol - padding;
+    }
+
+    assert((body + padding) % 8 == 0);
+    return (body + padding) / 8;
+}
+
+} // namespace access
+} // namespace vanetza