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_sink.cpp

@@ -0,0 +1,29 @@
+#include <gtest/gtest.h>
+#include <vanetza/common/byte_buffer_sink.hpp>
+#include <boost/iostreams/stream_buffer.hpp>
+#include <ostream>
+#include <string>
+
+using namespace vanetza;
+
+TEST(ByteBufferSink, write) {
+    ByteBuffer buf;
+    byte_buffer_sink sink(buf);
+    boost::iostreams::stream_buffer<byte_buffer_sink> stream(sink);
+    ASSERT_TRUE(buf.empty());
+
+    std::ostream os(&stream);
+    ASSERT_TRUE(os.good());
+
+    const std::string data("Hello World!");
+    os << data;
+    os.flush();
+    EXPECT_EQ(buf.size(), 12);
+
+    for (unsigned i = 0; i < buf.size(); ++i) {
+        EXPECT_EQ(buf[i], data[i]);
+    }
+
+    EXPECT_TRUE(os.good());
+}
+