chore(l0): scaffold C#9 netstandard2.0 core solution

Nightborn.L0.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.0.31903.59
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "src", "src", "{C3AD2FCF-F06D-48D2-8A84-2D8E73352FAD}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Nightborn.Core", "src\Nightborn.Core\Nightborn.Core.csproj", "{A1AFC50B-D6E5-4542-874A-2C911D6D3FE9}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{A1AFC50B-D6E5-4542-874A-2C911D6D3FE9}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{A1AFC50B-D6E5-4542-874A-2C911D6D3FE9}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{A1AFC50B-D6E5-4542-874A-2C911D6D3FE9}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{A1AFC50B-D6E5-4542-874A-2C911D6D3FE9}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(NestedProjects) = preSolution
+		{A1AFC50B-D6E5-4542-874A-2C911D6D3FE9} = {C3AD2FCF-F06D-48D2-8A84-2D8E73352FAD}
+	EndGlobalSection
+EndGlobal

src/Nightborn.Core/CoreModule.cs

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+namespace Nightborn.Core
+{
+    /// <summary>
+    /// Marker type for the L0 pure C# core assembly.
+    /// </summary>
+    public static class CoreModule
+    {
+    }
+}

src/Nightborn.Core/Nightborn.Core.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>netstandard2.0</TargetFramework>
+    <LangVersion>9.0</LangVersion>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>disable</ImplicitUsings>
+  </PropertyGroup>
+</Project>

src/Nightborn.Core/README.md

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+# Nightborn.Core (L0)
+
+Pure C# core logic layer for Nightborn.
+
+## Target
+- C#: 9.0
+- Target framework: `netstandard2.0`
+- Unity compatibility: usable from Unity projects that consume .NET Standard 2.0 libraries.
+
+## Current module folders
+- Combat
+- Enemy
+- Player
+- Progression
+- Persistence
+- Meta
+- Geometry