feat(serial): add serial port with output capture

src/GameBoy.Core/Serial/SerialPort.cs

@@ -0,0 +1,66 @@
+using System.IO;
+using System.Text;
+using GameBoy.Cpu;
+
+namespace GameBoy.Serial
+{
+    public sealed class SerialPort
+    {
+        private readonly InterruptController interrupts;
+        private readonly StringBuilder output = new StringBuilder();
+        private byte data;
+        private byte control;
+
+        public SerialPort(InterruptController interrupts)
+        {
+            this.interrupts = interrupts;
+        }
+
+        public string Output => output.ToString();
+
+        public byte Read(ushort address)
+        {
+            return (address & 0xFF) switch
+            {
+                0x01 => data,
+                0x02 => (byte)(control | 0x7E),
+                _ => 0xFF
+            };
+        }
+
+        public void Write(ushort address, byte value)
+        {
+            switch (address & 0xFF)
+            {
+                case 0x01:
+                    data = value;
+                    break;
+                case 0x02:
+                    control = value;
+                    if ((value & 0x81) == 0x81)
+                    {
+                        output.Append((char)data);
+                        data = 0xFF;
+                        control &= 0x7F;
+                        interrupts.Request(Interrupt.Serial);
+                    }
+                    break;
+            }
+        }
+
+        public void SaveState(BinaryWriter writer)
+        {
+            writer.Write(data);
+            writer.Write(control);
+            writer.Write(output.ToString());
+        }
+
+        public void LoadState(BinaryReader reader)
+        {
+            data = reader.ReadByte();
+            control = reader.ReadByte();
+            output.Clear();
+            output.Append(reader.ReadString());
+        }
+    }
+}