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>

CLAUDE.md

@@ -16,7 +16,8 @@ See README.md for the overview.
   or `$env:USERPROFILE\.platformio\penv\Scripts\pio.exe run` (PowerShell).
 - If an ESP32 build fails with `No module named 'intelhex'`, run
   `~/.platformio/penv/Scripts/python.exe -m pip install intelhex`.
-- After any change, build both targets: `pio run -e nano -e esp32`.
+- After any change, build both targets (`pio run`). After Nano changes, also run
+  `pio test -e native`.
 
 ## Constraints
 
@@ -55,9 +56,13 @@ See README.md for the overview.
 - The Nano only replies `READY` (at boot) and `ERR: ...`. There is no per-frame
   acknowledgement. Whether the Nano's TX is wired to the ESP's RX is unconfirmed, so every
   wait for a reply needs a timeout.
-- To test the parser without hardware, compile all `src/nano/*.cpp` files with g++ (Strawberry Perl
-  provides it at `C:\Strawberry\c\bin`) and mock headers for `Arduino.h`, `EEPROM.h` and
-  `BROSE9323.h`.
+- Unit tests: `pio test -e native` runs `test/test_nano/` on the PC with the fakes in
+  `test/mocks/` (Arduino, EEPROM, and a `BROSE9323` that records calls). It needs gcc on
+  PATH (Strawberry Perl provides it at `C:\Strawberry\c\bin`). Add a test for every
+  protocol change. The mocks only implement what the Nano code uses, so extend them when
+  new Arduino or driver functions are called.
+- The board environments have `test_ignore = *`, so test firmware is never uploaded. Plain
+  `pio test` runs nothing; always pass `-e native`.
 
 ## Git