fix: add MaterialSlotController.cs to UWP project Compile items

The legacy UWP .csproj uses explicit <Compile> entries rather than a glob.
MaterialSlotController.cs was added in the MaterialSlots commit but was not
included in the list, causing CS0246 on CI.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: arcadiogarcia

src/Combobulate/Combobulate.csproj

@@ -129,6 +129,7 @@
     <Compile Include="CombobulateSceneVisual.cs" />
     <Compile Include="GlobalUsings.cs" />
     <Compile Include="MaterialMode.cs" />
+    <Compile Include="MaterialSlotController.cs" />
     <Compile Include="Caching\MaterialResolver.cs" />
     <Compile Include="Caching\ObjCache.cs" />
     <Compile Include="Caching\ObjGeometry.cs" />