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/pki/psid_ssp.cpp

@@ -0,0 +1,40 @@
+#include "psid_ssp.hpp"
+#include <boost/algorithm/hex.hpp>
+#include <charconv>
+
+namespace vanetza
+{
+namespace pki
+{
+
+bool lexical_cast(const std::string& input, PsidSsp& permission)
+{
+    // split into the decimal PSID and the optional ':'-separated hex SSP.
+    const std::size_t colon = input.find(':');
+    const char* first = input.data();
+    const char* last = input.data() + (colon == std::string::npos ? input.size() : colon);
+
+    decltype(permission.psid) psid = 0;
+    auto [ptr, ec] = std::from_chars(first, last, psid);
+    if (ec != std::errc {} || ptr != last) {
+        // empty, contains a non-digit, or out of range
+        return false;
+    }
+
+    // found SSP part
+    if (colon != std::string::npos) {
+        try {
+            ByteBuffer ssp;
+            boost::algorithm::unhex(input.begin() + colon + 1, input.end(), std::back_inserter(ssp));
+            permission.ssp = std::move(ssp);
+        } catch (boost::algorithm::hex_decode_error&) {
+            return false;
+        }
+    }
+
+    permission.psid = psid;
+    return true;
+}
+
+} // namespace pki
+} // namespace vanetza