Add companion_radio_serial env for XIAO nRF52840 🤖🤖

[jpmartineau]

The XIAO nRF52840 leaves D6/D7 free once the Wio-SX1262 has taken the SPI pins, so it can carry the companion protocol on a hardware UART exactly like Xiao_S3_WIO_companion_radio_serial already does on the ESP32-S3 side. Two things were in the way.

HardwareSerial. The SERIAL_RX block declares HardwareSerial companion_serial(1) and calls setPins() on it. The Adafruit nRF52 core has no numbered constructor and HardwareSerial is abstract there, so the env doesn't compile:

error: cannot declare variable 'companion_serial' to be of abstract type 'HardwareSerial'
error: 'class HardwareSerial' has no member named 'setPins'

Serial1 is the concrete Uart instance the core always defines on the nRF52840, and Uart::setPins(pin_rx, pin_tx) takes the same two arguments in the same order as the ESP32 method, so a reference to it leaves the call site untouched.

I2C is mapped onto the same two pins. variants/xiao_nrf52 sets PIN_WIRE_SCL=D6 and PIN_WIRE_SDA=D7, and both XiaoNrf52Board::begin() and sensors.begin() call Wire on them. sensors.begin() runs after setup() has configured the UART, so TWIM takes the pads back as open-drain I2C and the link goes quiet with correct wiring and nothing logged anywhere — it took me a while to find. The new env unsets those two defines and points I2C at the internal IMU pins (16/17), which is where the variant header suggests they belong.

That remap is scoped to the new env with build_unflags rather than changed in the shared [Xiao_nrf52] flags, so the ble, usb, repeater, room server and kiss modem builds are unaffected. Nothing outside the new env changes.

Tested on a XIAO nRF52840 with a Wio-SX1262, D6/D7 wired to a Luckfox Lyra's UART1 (crossed, common ground, powered from 3V3). The node has been up on the mesh since I flashed it and answers a companion client on /dev/ttyS1 at 115200 — self info, contacts, config reads and writes, clock sync.

I also built Xiao_nrf52_companion_radio_usb and Xiao_S3_WIO_companion_radio_serial to check both sides of the new #if; neither regressed.

Context: I need this to use my Seeed nRF52840 in my Clockwork PicoCalc, using GPIO.