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/ModLoaderTests.cs

@@ -0,0 +1,114 @@
+using Xunit;
+
+namespace MrGameEng.Mods.Tests;
+
+public sealed class ModLoaderTests : IDisposable
+{
+    private readonly string _root = Directory.CreateTempSubdirectory("mrge-mods-tests-").FullName;
+
+    private string ModsRoot => Path.Combine(_root, "Mods");
+
+    public void Dispose() => Directory.Delete(_root, recursive: true);
+
+    /// <summary>Создаёт мод с About.json; зависимости — id модов, которые должны грузиться раньше.</summary>
+    private void WriteMod(string id, params string[] dependencies)
+    {
+        var aboutDir = Path.Combine(ModsRoot, id, "About");
+        Directory.CreateDirectory(aboutDir);
+        var deps = string.Join(", ", dependencies.Select(d => $"\"{d}\""));
+        File.WriteAllText(
+            Path.Combine(aboutDir, "About.json"),
+            $$"""{ "id": "{{id}}", "name": "{{id}} mod", "version": "1.0", "dependencies": [{{deps}}] }""");
+    }
+
+    [Fact]
+    public void Load_OrdersDependenciesFirst_ThenAlphabetically()
+    {
+        WriteMod("zebra", "Core");
+        WriteMod("apple", "Core");
+        WriteMod("Core");
+
+        var mods = ModLoader.Load(ModsRoot);
+
+        Assert.Equal(["Core", "apple", "zebra"], mods.Select(m => m.Id));
+    }
+
+    [Fact]
+    public void Load_MissingDependency_Throws()
+    {
+        WriteMod("orphan", "NoSuchMod");
+
+        var exception = Assert.Throws<InvalidDataException>(() => ModLoader.Load(ModsRoot));
+        Assert.Contains("NoSuchMod", exception.Message);
+    }
+
+    [Fact]
+    public void Load_CyclicDependency_Throws()
+    {
+        WriteMod("a", "b");
+        WriteMod("b", "a");
+
+        var exception = Assert.Throws<InvalidDataException>(() => ModLoader.Load(ModsRoot));
+        Assert.Contains("Cyclic", exception.Message);
+    }
+
+    [Fact]
+    public void Load_ActiveSubset_LoadsOnlyRequested()
+    {
+        WriteMod("Core");
+        WriteMod("extra", "Core");
+        WriteMod("unused");
+
+        var mods = ModLoader.Load(ModsRoot, ["extra", "Core"]);
+
+        Assert.Equal(["Core", "extra"], mods.Select(m => m.Id));
+    }
+
+    [Fact]
+    public void Load_IgnoresDirectoriesWithoutAbout()
+    {
+        WriteMod("Core");
+        Directory.CreateDirectory(Path.Combine(ModsRoot, "not-a-mod"));
+
+        var mod = Assert.Single(ModLoader.Load(ModsRoot));
+        Assert.Equal("Core", mod.Id);
+        Assert.Equal("Core mod", mod.Info.Name);
+    }
+
+    [Fact]
+    public void FindModsRoot_WalksUpFromNestedDirectory()
+    {
+        WriteMod("Core");
+        var nested = Path.Combine(_root, "bin", "Debug", "net8.0");
+        Directory.CreateDirectory(nested);
+
+        Assert.Equal(ModsRoot, ModLoader.FindModsRoot(nested));
+    }
+
+    [Fact]
+    public void ContentTree_LaterMod_OverridesSameRelativePath()
+    {
+        WriteMod("Core");
+        WriteMod("patch", "Core");
+        File.WriteAllText(CreateContentFile("Core", "Textures", "things/rock.png"), "core");
+        File.WriteAllText(CreateContentFile("Core", "Textures", "things/tree.png"), "core");
+        File.WriteAllText(CreateContentFile("patch", "Textures", "things/rock.png"), "patched");
+
+        var mods = ModLoader.Load(ModsRoot);
+        var tree = ModContentTree.Build(mods, "Textures", ".png");
+
+        Assert.Equal(2, tree.Files.Count);
+        Assert.True(tree.TryGet("things/rock.png", out var rock));
+        Assert.Equal("patch", rock.Mod.Id);
+        Assert.Equal("patched", File.ReadAllText(rock.FullPath));
+        Assert.True(tree.TryGet("things/tree.png", out var oak));
+        Assert.Equal("Core", oak.Mod.Id);
+    }
+
+    private string CreateContentFile(string modId, string folder, string relativePath)
+    {
+        var fullPath = Path.Combine(ModsRoot, modId, folder, relativePath);
+        Directory.CreateDirectory(Path.GetDirectoryName(fullPath)!);
+        return fullPath;
+    }
+}