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/ports/esp_idf/radio_c5.cmake

@@ -0,0 +1,27 @@
+# Kconfig hides this backend on other SoCs; this independent guard also rejects
+# forced/stale configurations. The private ABI is qualified per SDK version.
+if(NOT IDF_TARGET STREQUAL "esp32c5")
+    message(FATAL_ERROR "Integrated ITS-G5 radio requires IDF_TARGET=esp32c5")
+endif()
+if(NOT IDF_VERSION_MAJOR EQUAL 6 OR NOT IDF_VERSION_MINOR EQUAL 0 OR NOT IDF_VERSION_PATCH EQUAL 2)
+    message(FATAL_ERROR "C5 private radio ABI is pinned to ESP-IDF 6.0.2")
+endif()
+set(_otm_source "${VIDF_PORT}/third_party/otm/main/tx_custom.c")
+if(NOT EXISTS "${_otm_source}")
+    message(FATAL_ERROR "Initialize the OpenTrafficMap submodule before selecting the C5 radio")
+endif()
+file(READ "${_otm_source}" _otm)
+string(REPLACE "\r\n" "\n" _otm "${_otm}")
+string(SHA256 _otm_hash "${_otm}")
+if(NOT _otm_hash STREQUAL "cb1dccfef96912ca59275e8a9102f41f56925b94a19d4a4629082aaeb779be1b")
+    message(FATAL_ERROR "OpenTrafficMap transmitter differs from the reviewed source revision")
+endif()
+# Keep the upstream checkout immutable. The sole logic patch preserves the
+# actual driver submission result, which upstream discarded. ESP_OK still
+# means submission, not independent observation of transmission on air.
+string(REPLACE "            ieee80211_post_hmac_tx(eb);"
+               "            result = ieee80211_post_hmac_tx(eb);" _otm "${_otm}")
+set(_otm_generated "${CMAKE_CURRENT_BINARY_DIR}/otm_tx_custom.c")
+file(WRITE "${_otm_generated}" "#include <assert.h>\n#include <stddef.h>\n${_otm}\n_Static_assert(offsetof(x_ebuf_t, txdesc) == 0x38, \"ebuf txdesc offset\");\n_Static_assert(offsetof(x_eb_txdesc_t, rate) == 0x0c, \"txdesc rate offset\");\n")
+target_sources(${VIDF_TARGET} PRIVATE "${VIDF_PORT}/src/c5_radio.cpp" "${_otm_generated}")
+target_include_directories(${VIDF_TARGET} PRIVATE "${VIDF_PORT}/third_party/otm/main")