Add project files.

Core/Core.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Folder Include="Audio\" />
+    <Folder Include="Cpu\" />
+    <Folder Include="Interfaces\" />
+    <Folder Include="Io\" />
+    <Folder Include="Memory\" />
+  </ItemGroup>
+
+</Project>

Core/Cpu/RegisterPair.cs

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+using System.Runtime.InteropServices;
+
+namespace Core.Cpu
+{
+    [StructLayout(LayoutKind.Explicit)]
+    public struct RegisterPair
+    {
+        [FieldOffset(0)]
+        public ushort Word;
+
+        [FieldOffset(0)]
+        public byte Low;
+
+        [FieldOffset(1)]
+        public byte High;
+    }
+}