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/tools/socktap/certificate_validation_v3_strict.cpp

@@ -0,0 +1,14 @@
+#include "certificate_validation_v3.hpp"
+
+std::unique_ptr<CertificateValidationV3> create_certificate_validation_v3(
+    const boost::program_options::variables_map&, const vanetza::Runtime& runtime,
+    vanetza::PositionProvider& positioning,
+    const vanetza::security::v3::LocationChecker& location_checker,
+    vanetza::security::Backend&)
+{
+    return create_default_certificate_validation_v3(runtime, positioning, location_checker);
+}
+
+void add_certificate_validation_v3_options(boost::program_options::options_description&)
+{
+}