feat: draft board support for NANO_G2_ULTRA (#4)

[jamesarich]

Summary

Draft board port for NANO_G2_ULTRA (hwModel 18), closing the gap Meshtastic-Android's device_bootloader_ota_quirks.json flags in #4.

Every value in src/boards/nano_g2_ultra/board.h is sourced from something real, not guessed:

• MCU and button pin: read directly from meshtastic/firmware's variants/nrf52840/nano-g2-ultra/variant.h (nrf52840, one physical button at P1.06). No LED pin appears anywhere in that file or variant.cpp, so LEDS_NUMBER=0.
• Manufacturer/model/URL: firmware's platformio.ini (custom_meshtastic_display_name, etc.) and the vendor's own PlatformIO board manifest (datapartyjs/MeshTNC's boards/nano-g2-ultra.json), which also confirms this board reuses Adafruit's shared 0x239A VID rather than having a registered one of its own.

Two things are explicitly NOT confirmed, called out in both board.h and the new README section:

• USB_DESC_UF2_PID (0x00E2) was picked only to avoid colliding with every other USB_DESC_UF2_PID already in this repo under 0x239A — no dedicated allocation for this board was found anywhere.
• BUTTON_1 and BUTTON_2 both point at the same physical button (src/boards/boards.h hard-requires BUTTONS_NUMBER >= 2), mirroring wiscore_rak4631_board's own precedent for boards with only one real button — untested whether that's the right behavior on this specific hardware.

Why draft

This builds and links clean, but has not run on real hardware. Per this repo's own AGENTS.md guidance, a board port like this shouldn't be merged without that confirmation. @b8b8 — you mentioned on #4 that you have a few of these; if you're able to test, that would close the loop here. Marking as draft until then.

Test plan

• tools/build_all.py — all 15 boards (14 existing + this one) build and link clean
• Boots on real hardware
• UF2 mode (double-reset) and/or serial DFU mode actually work
• BLE OTA DFU actually works
• Confirm whether the shared BUTTON_1/BUTTON_2 pin causes any unexpected behavior
• Confirm or replace the guessed USB_DESC_UF2_PID

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[jamesarich]

@b8b8 can you check the nano_g2_ultra bootloader built off this branch pls 🙏