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/common/tests/byte_buffer.cpp

@@ -0,0 +1,35 @@
+#include <gtest/gtest.h>
+#include <vanetza/common/byte_buffer.hpp>
+#include <cstring>
+
+using namespace vanetza;
+
+struct A
+{
+    char b[10];
+};
+
+TEST(ByteBuffer, buffer_cast) {
+    ByteBuffer buf = { 'A', ' ', 't', 'e', 's', 't', ' ', 'b', 'u', 'f', 'f', 'e', 'r' };
+    ASSERT_GE(buf.size(), sizeof(A));
+    A* a = buffer_cast<A>(buf);
+    ASSERT_NE(nullptr, a);
+    EXPECT_EQ(a->b[0], 'A');
+    EXPECT_EQ(a->b[4], 's');
+    EXPECT_EQ(a->b[9], 'f');
+}
+
+TEST(ByteBuffer, buffer_copy) {
+    A a;
+    strcpy(a.b, "123456789");
+    auto copy = buffer_copy(a);
+    ASSERT_EQ(copy.size(), 10);
+    EXPECT_EQ(copy[0], '1');
+    EXPECT_EQ(copy[3], '4');
+    EXPECT_EQ(copy[8], '9');
+    EXPECT_EQ(copy[9], '\0');
+
+    a.b[0] = 'X';
+    EXPECT_EQ(copy[0], '1');
+}
+