Split Nano firmware into modules

- config.h: all constants and debug defines
- command_parser.h/.cpp: CommandParser class, one method per command
- settings.h/.cpp: EEPROM flip time storage
- main.cpp: setup/loop and the periodic full redraw only
- Full redraw now checks frameEnded() after the serial buffer is drained
- Drop the SPI/Adafruit_I2CDevice include workaround (not needed)

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

src/nano/README.md

@@ -6,6 +6,17 @@ The Nano is connected directly to the Brose panel electronics. It keeps an 84 ×
 buffer and takes drawing commands over serial (115200 baud). It has no clock or network
 logic.
 
+## Files
+
+| File | Contents |
+|------|----------|
+| `main.cpp` | `setup()` / `loop()`, feeds serial bytes to the parser, periodic full redraw |
+| `config.h` | All settings: debug options, display size, flip time limits, timings |
+| `command_parser.h/.cpp` | `CommandParser`: the serial protocol, one method per command |
+| `settings.h/.cpp` | Flip time stored in EEPROM |
+
+The display driver itself is in [lib/BROSE9323](../../lib/BROSE9323/README.md).
+
 ## Behaviour
 
 - **Boot:** the Nano loads the flip time from EEPROM, sets up the pins and sends `READY`.
@@ -24,7 +35,7 @@ logic.
 
 ### Debug options
 
-Uncomment these at the top of `main.cpp`:
+Uncomment these in `config.h`:
 
 | Define | Effect |
 |--------|--------|