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/geonet/next_hop.hpp

@@ -0,0 +1,92 @@
+#ifndef NEXT_HOP_HPP_ON0AKMBY
+#define NEXT_HOP_HPP_ON0AKMBY
+
+#include <vanetza/geonet/packet.hpp>
+#include <vanetza/geonet/pending_packet.hpp>
+#include <vanetza/geonet/pdu_variant.hpp>
+#include <vanetza/net/mac_address.hpp>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+/**
+ * NextHop is the result of GeoNet forwarding algorithms.
+ * It may convey a destination link-layer address or one of the states "discarded" and "buffered".
+ */
+class NextHop
+{
+public:
+    using Packet = PendingPacket<GbcPdu, const MacAddress&>;
+
+    NextHop();
+    NextHop(NextHop&&) = default;
+    NextHop& operator=(NextHop&&) = default;
+
+    /**
+     * Test if forwarding decision is to discard the packet.
+     * \return true if discarded
+     */
+    bool discarded() const;
+
+    /**
+     * Test if forwarding decided to buffer the packet.
+     * \return true if packet got buffered
+     */
+    bool buffered() const;
+
+    /**
+     * Test if stored packet and link-layer address are valid.
+     * \return if valid
+     */
+    bool valid() const;
+
+    /**
+     * Access stored link-layer address.
+     *
+     * The returned address is only meaningful if NextHop is valid.
+     * \return link-layer address
+     */
+    const MacAddress& mac() const;
+
+    /**
+     * Prepare for immediate packet transmission (not discarded, not buffered).
+     *
+     * valid() will return true after invocation of this method.
+     * \param packet the packet to be transmitted
+     * \param destination link-layer destination address
+     */
+    void transmit(Packet&& packet, const MacAddress& destination);
+
+    /**
+     * Set the NextHop state to discarded.
+     */
+    void discard();
+
+    /**
+     * Set the NextHop state to buffered.
+     */
+    void buffer();
+
+    /**
+     * Invoke further packet processing.
+     *
+     * It is safe to call this method though packet has been discarded or buffered.
+     * \return true if a valid packet (after previous transmit()) has been processed
+     */
+    bool process() &&;
+
+private:
+    enum class State { Valid, Discarded, Buffered };
+
+    State m_state;
+    MacAddress m_destination;
+    Packet m_packet;
+};
+
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* NEXT_HOP_HPP_ON0AKMBY */
+