Correct some analysis issues

Z80/Disassembler.cs

@@ -104,6 +104,8 @@ namespace Z80
 
         public string Disassemble(Z80 cpu, ushort pc)
         {
+            ArgumentNullException.ThrowIfNull(cpu);
+
             var opCode = this.Bus.Peek(pc);
 
             var decoded = cpu.GetDecodedOpCode(opCode);

Z80/Z80.FuseTest/RegisterState.cs

@@ -7,7 +7,7 @@ namespace Z80.FuseTest
     using Fuse;
     using System.Globalization;
 
-    internal class RegisterState : AbstractRegisterState, IRegisterState
+    internal sealed class RegisterState : AbstractRegisterState, IRegisterState
     {
         public int I { get; private set; } = -1;
 

Z80/Z80.FuseTest/TestRunner.cs

@@ -25,7 +25,7 @@ namespace Z80.FuseTest
         MEMPTR,
     }
 
-    internal class TestRunner : EightBit.Bus
+    internal sealed class TestRunner : EightBit.Bus
     {
         private readonly Test<RegisterState> test;
         private readonly Result<RegisterState> result;

Z80/Z80.FuseTest/TestSuite.cs

@@ -6,7 +6,7 @@ namespace Z80.FuseTest
 {
     using Fuse;
 
-    internal class TestSuite(string path)
+    internal sealed class TestSuite(string path)
     {
         private readonly Tests<RegisterState> tests = new(path + ".in");
         private readonly Results<RegisterState> results = new(path + ".expected");

Z80/Z80.Test/Board.cs

@@ -7,7 +7,7 @@ namespace Z80.Test
     using EightBit;
     using System.Globalization;
 
-    internal class Board : Bus
+    internal sealed class Board : Bus
     {
         private readonly Configuration configuration;
         private readonly Ram ram;