feat: add M5Stack Unit C6L variant support

[vistalba]

Summary

Adds full support for the M5Stack Unit C6L (ESP32-C6 + SX1262 LoRa), including PI4IO I/O expander for RF switch control and a custom minimal UITask for the 64x48 SSD1306 SPI display.

Changes

New variant: variants/m5stack_unit_c6l/

• Board class (UnitC6LBoard): PI4IO I/O expander (reset, direction, pull-up/pull-down), SPI bus management, display/RF switch init
• Hardware config (platformio.ini): corrected pins (NeoPixel TX on 2, I2C SDA=10/SCL=8, SPI shared with LoRa)
• DIO flash mode: board_build.flash_mode = dio — fixes bootloop on ESP32-C6 rev v0.2 (see Added flash_mode=dio to avoid boot loop when flashing using merge.bin #2570)
• 4 firmware environments: companion USB, companion BLE, repeater, room server

Display support (SSD1306SPIDisplay)

• New SPI-bus-sharing OLED driver in src/helpers/ui/SSD1306SPIDisplay.*
• Shares SPI bus with LoRa SX1262 via lazy init (spi_initialized flag)
• Custom minimal UITask in variants/m5stack_unit_c6l/companion/ — shows PIN, scrolling node name, frequency, unread count
• Fits 64x48 display (standard UITask overflows)

Shared fixes

• src/helpers/esp32/SerialBLEInterface.cpp: fixed include "esp_mac.h" → <esp_mac.h>
• src/helpers/ui/DisplayDriver.h: added MarqueeScroller struct

[MeshTast]

Thanks for supporting this device!! 👍

PS: Now I order 3 of these 😃

[Improver2000]

Wonderful! I'll switch then from Meshtastic to MeshCore with my m5stack. It's a nice simple starting device.

[vistalba]

I re-opend this as a new PR directly forked from this MeshCore repo.
You can find it here: #3100