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/otm_tx_custom.h

@@ -0,0 +1,23 @@
+#pragma once
+
+// Sourced from OpenTrafficMap's its-g5-receiver-firmware_txenabled, main/tx_custom.h
+// (https://codeberg.org/opentrafficmap/its-g5-receiver-firmware_txenabled, pinned commit
+// 674e34128279235ba34c9f8d778f43cf1d075397, no stated license). That project reverse-engineered
+// this ESP32-C5 ROM/libphy entry point -- it is not documented or supported by Espressif.
+// See experiments/evidence/c5-radio-characterization/phy-internals-investigation.md for what
+// this repository independently verified (by disassembling libphy.a) versus what is still an
+// unverified upstream guess.
+
+#include "esp_wifi.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+esp_err_t esp_wifi_80211_tx_custom(wifi_interface_t ifx, const void *buffer, int32_t len,
+                                   bool en_sys_seq, wifi_tx_rate_config_t *tx_rate_config,
+                                   wifi_band_t band, wifi_bandwidth_t bw);
+
+#ifdef __cplusplus
+}
+#endif