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/btp/header.hpp

@@ -0,0 +1,53 @@
+#ifndef HEADER_HPP_FNKGEM7C
+#define HEADER_HPP_FNKGEM7C
+
+#include <vanetza/common/byte_order.hpp>
+#include <vanetza/common/serialization.hpp>
+#include <vanetza/net/packet_variant.hpp>
+#include <cstdint>
+
+namespace vanetza
+{
+namespace btp
+{
+
+typedef uint16be_t port_type;
+
+// interactive packet transport
+struct HeaderA
+{
+    static constexpr std::size_t length_bytes = 4;
+
+    port_type destination_port;
+    port_type source_port;
+};
+
+static_assert(sizeof(HeaderA) == HeaderA::length_bytes, "Wrong size");
+
+void serialize(OutputArchive&, const HeaderA&);
+void deserialize(InputArchive&, HeaderA&);
+HeaderA parse_btp_a(ChunkPacket&);
+HeaderA parse_btp_a(CohesivePacket&);
+HeaderA parse_btp_a(PacketVariant&);
+
+// non-interactive packet transport
+struct HeaderB
+{
+    static constexpr std::size_t length_bytes = 4;
+
+    port_type destination_port;
+    uint16be_t destination_port_info;
+};
+
+static_assert(sizeof(HeaderB) == HeaderB::length_bytes, "Wrong size");
+
+void serialize(OutputArchive&, const HeaderB&);
+void deserialize(InputArchive&, HeaderB&);
+HeaderB parse_btp_b(ChunkPacket&);
+HeaderB parse_btp_b(CohesivePacket&);
+HeaderB parse_btp_b(PacketVariant&);
+
+} // namepsace btp
+} // namespace vanetza
+
+#endif /* HEADER_HPP_FNKGEM7C */