feat(cpu): add CB-prefixed opcode handling

src/GameBoy.Core/Cpu/Cpu.Cb.cs

@@ -0,0 +1,72 @@
+namespace GameBoy.Cpu
+{
+    public sealed partial class Cpu
+    {
+        private byte GetOperand(int index)
+        {
+            return index switch
+            {
+                0 => B,
+                1 => C,
+                2 => D,
+                3 => E,
+                4 => H,
+                5 => L,
+                6 => Read8(HL),
+                _ => A
+            };
+        }
+
+        private void SetOperand(int index, byte value)
+        {
+            switch (index)
+            {
+                case 0: B = value; break;
+                case 1: C = value; break;
+                case 2: D = value; break;
+                case 3: E = value; break;
+                case 4: H = value; break;
+                case 5: L = value; break;
+                case 6: Write8(HL, value); break;
+                default: A = value; break;
+            }
+        }
+
+        private void ExecuteCb()
+        {
+            byte op = Fetch8();
+            int index = op & 0x07;
+            int y = (op >> 3) & 0x07;
+            int block = op >> 6;
+
+            byte value = GetOperand(index);
+
+            switch (block)
+            {
+                case 0:
+                    byte rotated = y switch
+                    {
+                        0 => Rlc(value),
+                        1 => Rrc(value),
+                        2 => Rl(value),
+                        3 => Rr(value),
+                        4 => Sla(value),
+                        5 => Sra(value),
+                        6 => Swap(value),
+                        _ => Srl(value)
+                    };
+                    SetOperand(index, rotated);
+                    break;
+                case 1:
+                    BitTest(y, value);
+                    break;
+                case 2:
+                    SetOperand(index, ResetBit(y, value));
+                    break;
+                default:
+                    SetOperand(index, SetBit(y, value));
+                    break;
+            }
+        }
+    }
+}