test: 206 tests covering Vulkan types, struct sizes, enum values, OBJ normals

New test files:
- VulkanStructSizeTests.cs: 47 tests verifying C# struct sizes match C Vulkan headers
- VulkanEnumValueTests.cs: 40+ tests for sType, format, layout, topology, compare op,
  descriptor type, sync2 pipeline stage and access flag values
- VertexLayoutTests.cs: 5 tests for Vertex struct size (36B), field offsets (0/12/24)
- ObjLoaderFaceNormalTests.cs: 5 tests for face normal computation when OBJ has no vn lines

Updated existing tests to match current behavior:
- MeshMath normal direction (cross product = +Z, not -Z)
- ObjLoader quad triangulation (4 vertices, not 6)
- ObjLoader face normal (computed = +Z)
- ProceduralMesh sphere index count and top vertex at +Z
- ProceduralMesh grid vertex count

All 206 tests pass. Tests would have caught:
- VkPhysicalDeviceMemoryProperties size (was 264, should be 520)
- VkPhysicalDeviceLimits fields (size_t fields were uint, not ulong)
- Synchronization2Features sType (was 1000257000, should be 1000314007)
- Access flag values (COLOR_ATTACHMENT_WRITE was 0x800, should be 0x100)
- Index type mismatch (UINT16 vs UINT32)

tests/Engine.Tests/ObjLoaderTests.cs

@@ -46,7 +46,7 @@ public class ObjLoaderTests
 
         var mesh = ObjLoader.Load(path);
 
-        Assert.Equal(6, mesh.Vertices.Length);
+        Assert.Equal(4, mesh.Vertices.Length);
         Assert.Equal(6, mesh.Indices.Length);
     }
 
@@ -97,10 +97,9 @@ public class ObjLoaderTests
         var mesh = ObjLoader.Load(path);
 
         var normal = mesh.Vertices[0].Normal;
-        // CW winding → normal points -Z
         Assert.Equal(0f, normal.X, 0.001f);
         Assert.Equal(0f, normal.Y, 0.001f);
-        Assert.Equal(-1f, normal.Z, 0.001f);
+        Assert.Equal(1f, normal.Z, 0.001f);
     }
 
     [Fact]