Add FrSky BLE trainer (head tracker) support to the TX backpack

[Amigache]

Summary

Adds an opt-in BLE central to the TX backpack so a device advertising the
FrSky trainer service (0xFFF0 / 0xFFF6) - most notably a HeadTracker
in Wireless Transmitter (Head) mode - can feed the radio's trainer input
without the BTWifiModule hardware.

The backpack connects over BLE, parses the FrSky trainer frame (8 channels),
converts it to CRSF values and emits MSP_ELRS_BACKPACK_SET_PTR to the TX
module over the backpack UART. The module already understands that opcode
(HT Enabled / HT Start Channel = EdgeTX in the ELRS Lua), so no ExpressLRS
change is required for the core feature.

Although head tracking is the main use case, the implementation is generic:
any FrSky trainer source works. Hence the name FrSky BLE trainer
(BleTrainer).

Scope

• TX backpack only (ESP32-C3; the component is also valid for ESP32/S3).
  VRX and Timer backpacks are unchanged.
• Opt-in per target via HAS_BLE_TRAINER; currently enabled for
  ESP32C3_TX_Backpack_via_UART.
• The native (ESP-NOW/VRX) head-tracking path is untouched; a source selector
  chooses Auto / Native / BLE.

Changes

• src/BleTrainer.cpp + include/BleTrainer.h: NimBLE central (scan for
  0xFFF0, pair strongest RSSI, store MAC, reconnect on boot, Forget),
  per-notification FrSky parser (0x7E/0x7D byte-stuff, 0x80 + 8 channels,
  XOR CRC), PWM->CRSF conversion and SET_PTR emission.
• Steady 50 Hz republication of the last frame, so EdgeTX's trainer input does
  not time out between irregular BLE notifications.
• Connection params: 8 ms default interval (re-applied ~1.2 s after connect,
  since the trainer asks for its own first) and 4 s supervision timeout.
• BLE + MAVLink WiFi coexistence: esp_coex_preference_set(ESP_COEX_PREFER_WIFI)
  and reduced BLE TX power (P3) in MAVLINK_TX.
• Config (v6): trainerPeerMac / trainerPeerType / trainerIntervalMs /
  trainerSource (Auto / Native / BLE).
• WebUI tab FrSky BLE Trainer (status, paired MAC, interval, Forget, source)
  using the existing /config + callback() patterns; new POST /settrainer;
  JSON config.trainer.
• Long button press forgets the paired peer.
• HAS_BLE_TRAINER + h2zero/NimBLE-Arduino for the C3 TX backpack target.
• MAVLink WiFi mode is now persistent by telemMode (restored on every boot),
  so entering the updater no longer leaves the backpack without an AP.
• Dedup on a repeated MSP_ELRS_BACKPACK_CONFIG telemetry-mode message so the
  module cannot trigger a reboot loop.

Testing

Hardware: RadioMaster Nomad (ESP32-C3 TX backpack) + TX16S + Arduino Nano 33 BLE
running HeadTracker (Wireless Transmitter / Head); Mission Planner as GCS.

Validated:

• Trainer: linked:true, SET_PTR ~ 50/s (setptr 5917 -> 6119 in 4 s),
  EdgeTX trainer channels move.
• HT + MAVLink WiFi simultaneously: 10-min soak, 0 resets, 0 BLE
  disconnects, GCS telemetry continuous (packets_down ~ 55/s,
  overflows_down: 0).
• Module power 10/25/50/100/250 mW stable; cold boot at 250 mW; updater
  enter/exit returns to MAVLink WiFi.
• Build: 1.186.832 B (60.4 % of min_spiffs), RAM 55.980 B.

Notes for reviewers

• Naming is intentionally generic (FrSky BLE trainer): it implements the
  FrSky trainer profile, not head tracking specifically. Head tracker is
  documented as the main use case.
• NimBLE-Arduino stays on the 1.4.x branch because the repo pins
  espressif32@6.7.0 (Arduino-ESP32 2.0.x / IDF 4.4); 2.x needs a newer core.
• Config layout is additive within v6: no EEPROM migration and no need to
  re-pair.
• In Link Mode = MAVLINK the trainer needs a live RF link: without a linked
  receiver the module does not drain its UART FIFO. Worth a user-facing note;
  not a code issue.

Related

• Optional follow-up (separate ExpressLRS PR): a Forget trainer action in the
  Lua Backpack menu. The Lua builds the menu from the module's parameter
  structure, so only a module-side field plus a new MSP is needed. Not required
  for this PR.

[rotorman]

Tested with https://headtracker.gitbook.io/ (Arduino Nano 33 BLE Sense Rev.2 flashed with HT v2.4.0 code of https://github.com/headtracker/HeadTracker/releases ).