test(cartridge): add battery RAM persistence tests

tests/GameBoy.Core.Tests/BatteryRamTests.cs

@@ -0,0 +1,73 @@
+using Xunit;
+
+namespace GameBoy.Tests
+{
+    public sealed class BatteryRamTests
+    {
+        private static byte PatternByte(int bank, int offset)
+        {
+            return (byte)((bank * 31 + (offset & 0xFF)) ^ 0x5A);
+        }
+
+        private static void FillRam(Emulator emulator, int banks)
+        {
+            emulator.WriteMemory(0x0000, 0x0A);
+            for (int bank = 0; bank < banks; bank++)
+            {
+                emulator.WriteMemory(0x4000, (byte)bank);
+                for (int offset = 0; offset < 0x2000; offset++)
+                    emulator.WriteMemory((ushort)(0xA000 + offset), PatternByte(bank, offset));
+            }
+        }
+
+        private static void AssertRamMatches(Emulator emulator, int banks)
+        {
+            emulator.WriteMemory(0x0000, 0x0A);
+            for (int bank = 0; bank < banks; bank++)
+            {
+                emulator.WriteMemory(0x4000, (byte)bank);
+                for (int offset = 0; offset < 0x2000; offset++)
+                    Assert.Equal(PatternByte(bank, offset), emulator.ReadMemory((ushort)(0xA000 + offset)));
+            }
+        }
+
+        private static void RoundTrip(byte cartridgeType, byte ramSizeCode, int banks)
+        {
+            var source = new Emulator(TestRoms.BuildCartridge(cartridgeType, ramSizeCode));
+            Assert.True(source.HasBattery);
+
+            FillRam(source, banks);
+            byte[] saved = source.SaveRam();
+            Assert.Equal(banks * 0x2000, saved.Length);
+
+            var loaded = new Emulator(TestRoms.BuildCartridge(cartridgeType, ramSizeCode));
+            loaded.LoadRam(saved);
+            AssertRamMatches(loaded, banks);
+        }
+
+        [Fact]
+        public void Mbc1RamRoundTrips()
+        {
+            RoundTrip(0x03, 0x02, 1);
+        }
+
+        [Fact]
+        public void Mbc3RamRoundTrips()
+        {
+            RoundTrip(0x13, 0x03, 4);
+        }
+
+        [Fact]
+        public void Mbc5RamRoundTrips()
+        {
+            RoundTrip(0x1B, 0x03, 4);
+        }
+
+        [Fact]
+        public void PlainRomReportsNoBattery()
+        {
+            var emulator = new Emulator(TestRoms.BuildProgram());
+            Assert.False(emulator.HasBattery);
+        }
+    }
+}