fix: regenerate all OBJ files with verified CCW winding + remove grid

- pyramid.obj: verified CCW via cross-product for all 6 faces
- diamond.obj: verified CCW for all 8 octahedron faces
- torus.obj: verified CCW — normal points outward from torus center
- cone.obj: verified CCW for side faces (radial outward) and bottom (-Y)
- Removed Grid entity — balls were falling through it (no physics)
- Removed 'Grid' from castShadow exclusion list
- All objects now render correctly with CULL_MODE_BACK

src/Engine.Graphics.Vulkan/VulkanRenderer.cs

@@ -138,7 +138,7 @@ public sealed unsafe class VulkanRenderer : IRenderer, Engine.Graphics.IScreensh
             }
 
             var name = e.Name();
-            var castShadow = name != "Floor" && name != "Grid";
+            var castShadow = name != "Floor";
             drawCalls.Add((entry.vb.Buffer, entry.ib.Buffer, entry.indexCount, t.GetMatrix(), castShadow));
         });