Add native unit tests for the Nano firmware

- New native environment: pio test -e native
- test/mocks: fake Arduino core, EEPROM and recording BROSE9323 driver
- test/test_nano: 25 tests covering every command, bitmaps, bad input,
  flip time storage and the periodic full redraw
- Board environments ignore tests so they are never uploaded

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -38,6 +38,8 @@ lib/BROSE9323/          shared flipdot driver (used by both targets)
 src/nano/               Arduino Nano firmware
 src/esp32/              ESP32 firmware
   secrets.example.h     template for WiFi / MQTT credentials
+test/test_nano/         unit tests for the Nano firmware (run on the PC)
+test/mocks/             fake Arduino, EEPROM and display driver for the tests
 archive/                old, unused sketches kept for reference
 CLAUDE.md               project notes for Claude Code
 ```
@@ -64,8 +66,12 @@ pio run -e nano -t upload        # flash the Nano
 pio run -e esp32 -t upload       # flash the ESP32 over the air
 pio run -e esp32_usb -t upload   # flash the ESP32 over USB
 pio device monitor -e nano       # serial monitor
+pio test -e native               # run the Nano unit tests on the PC
 ```
 
+The unit tests need `gcc`/`g++` on your PATH. On Windows you can get it from
+[MSYS2](https://www.msys2.org/) or Strawberry Perl.
+
 In VS Code you can also pick the environment in the PlatformIO status bar.
 
 > **Tip:** The ESP32 → Nano wire goes to the Nano's RX pin, which the Nano's USB chip