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/tests/data_rates.cpp

@@ -0,0 +1,19 @@
+#include <gtest/gtest.h>
+#include <vanetza/access/data_rates.hpp>
+
+using namespace vanetza::access;
+
+TEST(DataRates, bytes_per_second)
+{
+    EXPECT_EQ(G5_3Mbps.bytes_per_second(), 3 * 1000 * 1000 / 8);
+    EXPECT_EQ(G5_6Mbps.bytes_per_second(), 6 * 1000 * 1000 / 8);
+    EXPECT_EQ(G5_12Mbps.bytes_per_second(), 12 * 1000 * 1000 / 8);
+}
+
+TEST(DataRates, data_length)
+{
+    const std::size_t psdu_bytes = 385;
+    const std::size_t data_bytes = G5_6Mbps.data_length(psdu_bytes);
+    EXPECT_GE(data_bytes, psdu_bytes);
+    EXPECT_LT(data_bytes, psdu_bytes + 12);
+}