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/byte_buffer_source.cpp

@@ -0,0 +1,30 @@
+#include "byte_buffer_source.hpp"
+#include <algorithm>
+
+namespace vanetza
+{
+
+byte_buffer_source::byte_buffer_source(const ByteBuffer& buffer) :
+    m_begin(buffer.cbegin()), m_end(buffer.cend()) {}
+
+byte_buffer_source::byte_buffer_source(range r) :
+    m_begin(r.begin()), m_end(r.end()) {}
+
+byte_buffer_source::byte_buffer_source(iterator begin, iterator end) :
+    m_begin(begin), m_end(end) {}
+
+std::streamsize byte_buffer_source::read(char_type* buf, std::streamsize n)
+{
+    if (m_begin == m_end) {
+        return -1;
+    } else {
+        std::streamsize remaining = std::distance(m_begin, m_end);
+        std::streamsize consume = std::min(remaining, n);
+        std::copy(m_begin, m_begin + consume, buf);
+        std::advance(m_begin, consume);
+        return consume;
+    }
+}
+
+} // namespace vanetza
+