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>

test/mocks/EEPROM.h

@@ -0,0 +1,30 @@
+// In-memory EEPROM for the native tests (starts erased, all 0xFF like a new chip)
+#pragma once
+
+#include <cstdint>
+#include <cstring>
+
+class FakeEEPROM
+{
+public:
+	uint8_t mem[1024];
+
+	FakeEEPROM() { erase(); }
+	void erase() { memset(mem, 0xFF, sizeof(mem)); }
+
+	template <typename T>
+	T &get(int addr, T &t)
+	{
+		memcpy(&t, mem + addr, sizeof(T));
+		return t;
+	}
+
+	template <typename T>
+	const T &put(int addr, const T &t)
+	{
+		memcpy(mem + addr, &t, sizeof(T));
+		return t;
+	}
+};
+
+inline FakeEEPROM EEPROM;