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/exception.hpp

@@ -0,0 +1,37 @@
+#pragma once
+
+#include <stdexcept>
+#include <string>
+
+namespace vanetza
+{
+namespace pki
+{
+
+class DecodingFailure : public std::runtime_error
+{
+    using std::runtime_error::runtime_error;
+};
+
+class VerificationFailure : public std::runtime_error
+{
+    using std::runtime_error::runtime_error;
+};
+
+class UsageError : public std::runtime_error
+{
+public:
+    UsageError(const std::string& problem) : std::runtime_error(problem) {}
+    UsageError(const std::string& problem, const std::string& remedy) :
+        std::runtime_error(problem), m_remedy(remedy)
+    {
+    }
+
+    const std::string& remedy() const { return m_remedy; }
+
+private:
+    std::string m_remedy;
+};
+
+} // namespace pki
+} // namespace vanetza