[RP2350B] Add MokyaLora board variant definition

[tengigabytes]

Goal

Add the RP2350B-MokyaLora board variant so Meshtastic can target this hardware.

Files to create

• variants/rp2350b-mokya/variant.h — pin definitions
• variants/rp2350b-mokya/platformio.ini — build environment (Arduino-Pico, RP2350B)

Key pin assignments (from mcu-gpio-allocation.md Rev 5.2)

Signal	GPIO
SPI1 MISO/nCS/SCK/MOSI (SX1262)	24 / 25 / 26 / 27
LORA_BUSY / LORA_DIO1	28 / 29
LORA_nRST	23
I2C0 SDA/SCL (sensor bus)	34 / 35
I2C1 SDA/SCL (power+backlight)	6 / 7
DBG UART TX/RX	2 / 3
PWR_BTN (DORMANT wakeup)	33

Acceptance criteria

• pio run -e rp2350b-mokya compiles without errors
• All GPIO constants match KiCad schematic Rev 5.2