Sign ITS messages on the ESP32-C5 with vanetza-idf, over USB or BLE

obu-firmware is now a port of the colleague's standalone VRU station
(microbu-esp32c5/firmware, kept beside this repository and gitignored): the
vanetza-idf C-ITS stack with the TS 103 097 security entity, credentials in
NVS, the station-link v1 protocol over the native USB port (frame type 0x10
in the existing 0xAA55 framing) and over a BLE GATT peripheral, and its
ITS-G5 radio adapter. The phone still builds CAM and VAM; the board adds
GeoNetworking/BTP and signs with the provisioned authorization ticket. The
private key never leaves the board. Builds with ESP-IDF 6.0.2 only, which
vanetza-idf pins for the radio's private driver ABI. The previous C firmware
stays on disk unbuilt; a full-flash backup of the bench board is kept in
firmware-backups/ (gitignored).

Changed against the colleague's firmware, marked MicrOBU: in the sources:
- Reception unchanged for the app. vanetza-idf drops what it cannot verify
  (unsigned traffic, every RSU), so each captured frame also goes through the
  previous gn_unwrap.c and reaches the phone as link opcode V2X_RX (0x85),
  whose body is the old SERIAL_MSG_V2X_RX payload.
- Unsigned transmission still possible, with the previous geonet.c header;
  the phone chooses per message.
- Console on UART0 (CH343 port); the native USB port carries only link frames.
- BLE advertising pauses while the USB link is in use: BLE and ITS-G5 share
  one RF front end.
- NVS 80 KB (app at 0x20000). At 24 KB, with Wi-Fi settings the previous
  firmware left behind, the BLE bond could not be stored and the phone had to
  pair on every connection.
- Bench fixes: the radio queue is drained before the first PoTi (no RX and
  ~177 queue drops before); the station loop waited pdMS_TO_TICKS(5) = 0
  ticks at 100 Hz and starved the idle task; the 2.4 KB RX capture buffer is
  off the Wi-Fi task stack; BLE notifications longer than the MTU are dropped
  instead of cut short, MTU 517; serial writes are skipped with no USB host.
- Manual country policy and TX-power read-back from the previous radio setup;
  logs for BLE encryption changes and the number of stored bonds.

Verified on the bench board (COM3) with the phone over USB and BLE: CAM and
VAM, signed and unsigned, go out; reception of the sim car and the RSU's
CAM/SPATEM/MAPEM continues; the board survives app restarts and reconnects.
See docs/06-signed-its-vam-ble.md.

obu-firmware/main/link_service.hpp

@@ -0,0 +1,51 @@
+#pragma once
+// Dispatches station-link messages (phone -> micrOBU) to the station and sends the
+// micrOBU -> phone messages (results, indications, identifier-change events, status).
+#include "link_protocol.hpp"
+#include "station.hpp"
+
+namespace microbu {
+
+/// @brief Represents the transport mechanism for link messages. Either BLE or Serial
+enum class LinkTransport : std::uint8_t { serial, ble };
+
+class LinkService {
+public:
+    /// @brief Binds the service to a station and subscribes to its indication/id-event callbacks.
+    /// @param station Station to bind to; must outlive the LinkService.
+    explicit LinkService(Station& station);
+    /// @brief Decodes and dispatches one received message; call from the station task.
+    /// @param message Raw message bytes as received from the transport.
+    /// @param origin Transport the message arrived on; replies go back on the same transport.
+    void handle(const link::Bytes& message, LinkTransport origin);
+    /// @brief Periodic work: broadcasts STATUS once a second.
+    void tick();
+private:
+    /// @brief Encodes and writes one message on the given transport.
+    /// @param transport Transport to write to.
+    /// @param opcode Message opcode.
+    /// @param sequence Sequence number to stamp on the header.
+    /// @param body Encoded message body.
+    void send(LinkTransport transport, link::Opcode opcode, std::uint16_t sequence, const link::Bytes& body);
+    /// @brief Sends a message on serial, and also on BLE when a phone is connected.
+    /// @param opcode Message opcode.
+    /// @param sequence Sequence number to stamp on the header.
+    /// @param body Encoded message body.
+    void broadcast(link::Opcode opcode, std::uint16_t sequence, const link::Bytes& body);
+    /// @brief Sends a RESULT reply to the message's origin transport.
+    /// @param origin Transport to reply on.
+    /// @param sequence Sequence number of the message being replied to.
+    /// @param code Result code to report.
+    /// @param detail Optional result detail bytes.
+    void reply(LinkTransport origin, std::uint16_t sequence, link::Code code, const link::Bytes& detail = {});
+    /// @brief Builds a STATUS snapshot from the station and transport counters.
+    /// @return Current status.
+    link::Status status();
+    Station& station_;
+    std::uint16_t own_sequence_ = 0;
+    link::Bytes bundle_; // credentials being provisioned
+    std::uint16_t bundle_total_ = 0;
+    std::int64_t last_status_us_ = 0;
+};
+
+} // namespace microbu