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/asn1/memory.c

@@ -0,0 +1,27 @@
+#include <stdlib.h>
+
+static size_t max_size = 4096;
+
+void* vanetza_asn1c_malloc(size_t size)
+{
+    if (size <= max_size)
+        return malloc(size);
+    else
+        return NULL;
+}
+
+void* vanetza_asn1c_calloc(size_t nmemb, size_t size)
+{
+    if (nmemb * size <= max_size)
+        return calloc(nmemb, size);
+    else
+        return NULL;
+}
+
+void* vanetza_asn1c_realloc(void* ptr, size_t size)
+{
+    if (size <= max_size)
+        return realloc(ptr, size);
+    else
+        return NULL;
+}