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/duty_cycle_permit.hpp

@@ -0,0 +1,30 @@
+#ifndef DUTY_CYCLE_PERMIT_HPP_9QUTPOPH
+#define DUTY_CYCLE_PERMIT_HPP_9QUTPOPH
+
+#include <vanetza/common/unit_interval.hpp>
+
+namespace vanetza
+{
+namespace dcc
+{
+
+/**
+ * Interface for controlling channel usage by duty cycle limits
+ */
+class DutyCyclePermit
+{
+public:
+    /**
+     * Get allowed channel occupancy for local station in current time window
+     * \return permitted duty cycle
+     */
+    virtual UnitInterval permitted_duty_cycle() const  = 0;
+
+    virtual ~DutyCyclePermit() = default;
+};
+
+} // namespace dcc
+} // namespace vanetza
+
+#endif /* DUTY_CYCLE_PERMIT_HPP_9QUTPOPH */
+