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/BROSE9323.h

@@ -0,0 +1,58 @@
+// Fake display driver for the native tests: records every call instead of flipping dots
+#pragma once
+
+#include <string>
+#include <vector>
+
+#include "Arduino.h"
+
+struct DisplayCall
+{
+	std::string what;
+	int a, b, c;
+};
+
+class BROSE9323
+{
+public:
+	std::vector<DisplayCall> calls;
+	bool direct = false;
+
+	BROSE9323(uint8_t, uint8_t, uint8_t, uint16_t) {}
+
+	void begin() { log("begin"); }
+	void display(bool force = false) { log("display", force); }
+	void drawPixel(int16_t x, int16_t y, uint16_t color) { log(direct ? "directPixel" : "pixel", x, y, color); }
+	void fillScreen(uint16_t color) { log("fill", color); }
+	void setDirect(bool d) { direct = d; }
+	void setTiming(uint16_t t) { log("timing", t); }
+	void printBuffer() { log("printBuffer"); }
+	void setCursor(int16_t x, int16_t y) { log("cursor", x, y); }
+	void setTextColor(uint16_t color, uint16_t background) { log("textColor", color, background); }
+	void setTextSize(uint8_t size) { log("textSize", size); }
+	size_t write(uint8_t ch)
+	{
+		log("write", ch);
+		return 1;
+	}
+	void println() { log("newline"); }
+
+	// Helpers for the tests
+	int count(const std::string &what) const
+	{
+		int n = 0;
+		for (const DisplayCall &call : calls)
+			n += call.what == what;
+		return n;
+	}
+	const DisplayCall *last(const std::string &what) const
+	{
+		for (auto it = calls.rbegin(); it != calls.rend(); ++it)
+			if (it->what == what)
+				return &*it;
+		return nullptr;
+	}
+
+private:
+	void log(const char *what, int a = 0, int b = 0, int c = 0) { calls.push_back({what, a, b, c}); }
+};