Add project files.

Harmonia.Core/Class1.cs

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+namespace Harmonia.Core
+{
+    public class Class1
+    {
+
+    }
+}

Harmonia.Core/Harmonia.Core.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

Harmonia.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.12.35728.132 d17.12
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Harmonia.Core", "Harmonia.Core\Harmonia.Core.csproj", "{BC40B066-CD33-4A9E-A470-A0F9216D1822}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{BC40B066-CD33-4A9E-A470-A0F9216D1822}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{BC40B066-CD33-4A9E-A470-A0F9216D1822}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{BC40B066-CD33-4A9E-A470-A0F9216D1822}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{BC40B066-CD33-4A9E-A470-A0F9216D1822}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+EndGlobal