feat(a1): .NET 8 solution scaffold + Model + OpcodeTable loader

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

engine/Age.Engine.Tests/Age.Engine.Tests.csproj

@@ -0,0 +1,27 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
+    <PackageReference Include="xunit" Version="2.5.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.5.3" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Age.Engine\Age.Engine.csproj" />
+  </ItemGroup>
+
+</Project>

engine/Age.Engine.Tests/OpcodeTableTests.cs

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+using Age.Engine.Sys4;
+using Xunit;
+
+public class OpcodeTableTests
+{
+    [Fact]
+    public void LoadsAll248FromJson()
+    {
+        var t = OpcodeTableJson.Load(Paths.OpcodesJson);
+        Assert.Equal(248, t.Count);
+        Assert.True(t.TryGet(0x55, out var label, out var argc));
+        Assert.Equal("mov", label);
+        Assert.Equal(2, argc);
+        Assert.Equal("u0041BEB0", t.Label(0x90));
+        Assert.Equal(7, t.Argc(0x90));
+        Assert.Equal(-1, t.Argc(0x9999)); // absent -> -1
+    }
+}