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/channel_load.hpp

@@ -0,0 +1,40 @@
+#ifndef CHANNEL_LOAD_HPP_D1JOCNLP
+#define CHANNEL_LOAD_HPP_D1JOCNLP
+
+#include <vanetza/common/unit_interval.hpp>
+
+namespace vanetza
+{
+namespace dcc
+{
+
+class ChannelLoad : public UnitInterval
+{
+public:
+    using UnitInterval::UnitInterval;
+    ChannelLoad() = default;
+    ChannelLoad(const UnitInterval&);
+
+    /**
+     * Create ChannelLoad from rational probes
+     * \see ChannelLoad::create_from_probes
+     *
+     * \param probes_busy number of probes above busy threshold
+     * \param probes_total total number of probes
+     */
+    ChannelLoad(unsigned probes_busy, unsigned probes_total);
+
+    /**
+     * Calculate UnitInterval representing ChannelLoad from rational probes
+     * \param probes_busy number of probes above busy threshold
+     * \param probes_total total number of probes
+     * \return interval representing channel load (capped if probes_total < probes_busy)
+     */
+    static UnitInterval create_from_probes(unsigned probes_busy, unsigned probes_total);
+};
+
+} // namespace dcc
+} // namespace vanetza
+
+#endif /* CHANNEL_LOAD_HPP_D1JOCNLP */
+