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>

platformio.ini

@@ -3,11 +3,13 @@
 ;   nano       Arduino Nano that drives the Brose flipdot panels directly
 ;   esp32      ESP32 "brain" (WiFi, NTP clock, Home Assistant) - OTA upload
 ;   esp32_usb  Same ESP32 firmware, flashed over USB (first flash / recovery)
+;   native     Unit tests for the Nano firmware, run on the PC (needs gcc on PATH)
 ;
 ; Build all:        pio run
 ; Build one:        pio run -e nano
 ; Upload one:       pio run -e nano -t upload
 ; Serial monitor:   pio device monitor -e nano
+; Run tests:        pio test -e native
 ;
 ; https://docs.platformio.org/page/projectconf.html
 
@@ -19,6 +21,8 @@ framework = arduino
 monitor_speed = 115200
 lib_deps =
 	adafruit/Adafruit GFX Library@^1.10.12
+; Tests only run in the native environment, never on the boards
+test_ignore = *
 
 ; ---------------------------------------------------------------------------
 ; Arduino Nano - display driver
@@ -57,3 +61,17 @@ monitor_port = COM11
 extends = env:esp32
 upload_protocol = esptool
 upload_port = COM11
+
+; ---------------------------------------------------------------------------
+; Native - unit tests for the Nano firmware on the PC (test/test_nano)
+; The Arduino core, EEPROM and the display driver are replaced by test/mocks.
+; ---------------------------------------------------------------------------
+[env:native]
+platform = native
+framework =
+lib_deps =
+lib_ignore = BROSE9323
+test_ignore =
+test_build_src = yes
+build_src_filter = +<nano/>
+build_flags = -std=gnu++17 -I test/mocks