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/dcc/transmission.cpp

@@ -0,0 +1,33 @@
+#include "transmission.hpp"
+#include <chrono>
+
+namespace vanetza
+{
+namespace dcc
+{
+
+Clock::duration Transmission::channel_occupancy() const
+{
+    using namespace std::chrono;
+
+    // assume 6 Mbps as default data rate
+    const access::DataRateG5* rate = data_rate() ? data_rate() : &access::G5_6Mbps;
+
+    // PHY
+    static const auto phy_preamble = microseconds(32);
+    static const auto phy_signal = microseconds(8);
+
+    // MAC
+    static const std::size_t bytes_epd = 2; // EtherType Protocol Discrimination (no LLC!)
+    static const std::size_t bytes_mac = 34; // 802.11 MAC header
+
+    const std::size_t bytes = rate->data_length(body_length() + bytes_epd + bytes_mac);
+    const double seconds_per_byte = 1.0 / (rate->bytes_per_second());
+    const duration<double> data_duration { bytes * seconds_per_byte };
+
+    return phy_preamble + phy_signal + duration_cast<microseconds>(data_duration);
+}
+
+
+} // namespace dcc
+} // namespace vanetza