Add MrGameEng.Mods: mod loading, JSON defs and localization

Mods are folders with About/About.json metadata; ModLoader resolves a
deterministic load order (dependencies first, ties alphabetical) and
later mods override earlier ones everywhere.

DefDatabase loads JSON def files ({ "type", "defs": [...] }) into
game-registered Def subclasses, with parent inheritance (own fields on
top of the parent's, nested values replaced whole), abstract parents
and full replacement of same-named defs by later mods.

LanguageManager loads Languages/<code>/*.json flat key-string maps,
switches language at runtime and falls back current -> default -> key.

ModContentTree merges one content folder across mods by relative path;
AtlasBuilder gains an explicit-sources Build overload so a merged
texture tree can be packed incrementally at game start.

The LittleSim game now ships its entire content as the Core mod,
demonstrating the module end to end.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/MrGameEng.Mods.Tests/MrGameEng.Mods.Tests.csproj

@@ -0,0 +1,19 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <OutputType>Exe</OutputType>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" />
+    <PackageReference Include="xunit.v3" />
+    <PackageReference Include="xunit.runner.visualstudio" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\MrGameEng.Mods\MrGameEng.Mods.csproj" />
+  </ItemGroup>
+
+</Project>