build(io): drop OpenNest.IO's reference to the engine

IO only needed Engine for the best-fit cache, which now goes through
IBestFitStore. Removing the reference stops every IO consumer pulling in
the engine and ONNX Runtime. A guard test keeps the assembly from
referencing it again; IO.Tests references Engine itself for its whole-job
round-trip tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.IO.Tests/IoDependencyTests.cs

@@ -0,0 +1,12 @@
+namespace OpenNest.IO.Tests;
+
+public class IoDependencyTests
+{
+    [Fact]
+    public void IoAssembly_DoesNotReferenceTheEngine()
+    {
+        var references = typeof(NestReader).Assembly.GetReferencedAssemblies().Select(a => a.Name);
+
+        Assert.DoesNotContain("OpenNest.Engine", references);
+    }
+}

OpenNest.IO.Tests/OpenNest.IO.Tests.csproj

@@ -12,5 +12,7 @@
     <PackageReference Include="xunit.runner.visualstudio" Version="2.5.3" />
     <Using Include="Xunit" />
     <ProjectReference Include="../OpenNest.IO/OpenNest.IO.csproj" />
+    <!-- Some round-trip tests drive whole jobs; OpenNest.IO itself must not reference the engine. -->
+    <ProjectReference Include="../OpenNest.Engine/OpenNest.Engine.csproj" />
   </ItemGroup>
 </Project>

OpenNest.IO/OpenNest.IO.csproj

@@ -9,7 +9,6 @@
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\OpenNest.Core\OpenNest.Core.csproj" />
-    <ProjectReference Include="..\OpenNest.Engine\OpenNest.Engine.csproj" />
     <PackageReference Include="ACadSharp" Version="3.1.32" />
     <PackageReference Include="ClosedXML" Version="0.104.2" />
   </ItemGroup>