test(pipeline): MP1b - AcDream.Content.Tests scaffold

New xunit test project for the Content layer (rule 6: tests live in the
project matching the layer under test). ProjectReference to
AcDream.Content; TreatWarningsAsErrors + LangVersion latest per MP1b's
csproj requirements. Registered in AcDream.slnx. Placeholder PakKeyTests
smoke test becomes the real Task 2 suite next.

tests/AcDream.Content.Tests/AcDream.Content.Tests.csproj

@@ -0,0 +1,27 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+    <LangVersion>latest</LangVersion>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\AcDream.Content\AcDream.Content.csproj" />
+  </ItemGroup>
+
+</Project>

tests/AcDream.Content.Tests/PakKeyTests.cs

@@ -0,0 +1,14 @@
+namespace AcDream.Content.Tests;
+
+/// <summary>
+/// Task 1 scaffold smoke test — becomes the real PakKey test suite in Task 2.
+/// </summary>
+public class PakKeyTests {
+    [Fact]
+    public void Smoke_ProjectReferencesContentAssembly() {
+        // Any type from AcDream.Content resolves — proves the ProjectReference
+        // and test-project wiring are correct before Task 2 adds real coverage.
+        var key = new TextureKey();
+        Assert.Equal(0u, key.SurfaceId);
+    }
+}