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/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);
+    }
+}