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/signed_builder.hpp

@@ -0,0 +1,47 @@
+#pragma once
+
+#include "keys.hpp"
+#include "signed_data.hpp"
+#include <vanetza/common/byte_buffer.hpp>
+#include <vanetza/security/hash_algorithm.hpp>
+#include <cstdint>
+
+namespace vanetza
+{
+namespace pki
+{
+
+class Certificate;
+class SecurityModule;
+
+using security::HashAlgorithm;
+
+// TS 103 097 protocol version that we emit on every constructed EtsiTs103097Data.
+inline constexpr std::uint8_t ieee1609dot2_protocol_version = 3;
+
+/**
+ * Build an EtsiTs103097Data-Signed envelope carrying `payload` as inline
+ * `unsecuredData`, signed with `signing_key`. PSID is fixed to SCR.
+ *
+ * \param signer_cert selects the SignerIdentifier variant per IEEE 1609.2 §5.3.1:
+ *   nullptr  → signer = self    (hash of signer is hash of empty string)
+ *   non-null → signer = digest  with HashedId8(cert); hash of signer is hash
+ *              of the OER-encoded signer certificate.
+ */
+SignedData create_signed(const ByteBuffer& payload, SecurityModule& security,
+    const PublicKey& signing_key, HashAlgorithm hash_algo, const Certificate* signer_cert);
+
+/**
+ * Build an EtsiTs103097Data-SignedExternalPayload (TS 102 941 §6.2.3.3.1) used
+ * for the EC-signed proof inside an AT request: SignedData with
+ * `extDataHash = hash_algo(external_payload)` and no inline data.
+ *
+ * The same `hash_algo` covers both the external-payload hash placed in
+ * extDataHash and the signature hash chain. Other parameters mirror
+ * create_signed(); PSID is fixed to SCR.
+ */
+SignedData create_external_signed(const ByteBuffer& external_payload, SecurityModule& security,
+    const PublicKey& signing_key, HashAlgorithm hash_algo, const Certificate* signer_cert);
+
+} // namespace pki
+} // namespace vanetza