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/Arduino.h

@@ -0,0 +1,55 @@
+// Minimal Arduino API for running the Nano code on the PC (native test environment)
+#pragma once
+
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+#include <deque>
+#include <string>
+
+#define HEX 16
+
+class __FlashStringHelper;
+#define F(s) ((const __FlashStringHelper *)(s))
+
+// Time is controlled by the tests
+inline uint32_t fake_millis = 0;
+inline uint32_t millis() { return fake_millis; }
+inline void delay(uint32_t ms) { fake_millis += ms; }
+
+// Serial port: tests push bytes into `in` and read replies from `out`
+class FakeSerial
+{
+public:
+	std::deque<char> in;
+	std::string out;
+
+	void begin(unsigned long) {}
+	int available() { return in.size(); }
+	int read()
+	{
+		if (in.empty())
+			return -1;
+		char c = in.front();
+		in.pop_front();
+		return (uint8_t)c;
+	}
+	void flush() {}
+
+	void print(const __FlashStringHelper *s) { out += (const char *)s; }
+	void print(const char *s) { out += s; }
+	void print(int v, int base = 10)
+	{
+		char buf[16];
+		snprintf(buf, sizeof(buf), base == HEX ? "%X" : "%d", v);
+		out += buf;
+	}
+	void println(const __FlashStringHelper *s) { print(s); out += "\n"; }
+	void println(const char *s) { print(s); out += "\n"; }
+	void println(unsigned long v) { out += std::to_string(v) + "\n"; }
+};
+
+using Print = FakeSerial;
+
+inline FakeSerial Serial;