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/VertexLayoutTests.cs

@@ -0,0 +1,54 @@
+using System.Runtime.InteropServices;
+using Engine.Core;
+
+namespace Engine.Tests;
+
+/// <summary>
+/// Verifies the Vertex struct layout matches what the Vulkan vertex input description expects.
+/// The struct is { Vector3 Position, Vector3 Color, Vector3 Normal } = 9 floats = 36 bytes.
+/// </summary>
+public unsafe class VertexLayoutTests
+{
+    [Fact]
+    public void Vertex_Is_36_Bytes()
+    {
+        Assert.Equal(36, sizeof(Vertex));
+    }
+
+    [Fact]
+    public void Vertex_Has_Three_Vector3_Fields()
+    {
+        var v = new Vertex(
+            new System.Numerics.Vector3(1, 2, 3),
+            new System.Numerics.Vector3(4, 5, 6),
+            new System.Numerics.Vector3(7, 8, 9));
+
+        Assert.Equal(1f, v.Position.X);
+        Assert.Equal(2f, v.Position.Y);
+        Assert.Equal(3f, v.Position.Z);
+        Assert.Equal(4f, v.Color.X);
+        Assert.Equal(5f, v.Color.Y);
+        Assert.Equal(6f, v.Color.Z);
+        Assert.Equal(7f, v.Normal.X);
+        Assert.Equal(8f, v.Normal.Y);
+        Assert.Equal(9f, v.Normal.Z);
+    }
+
+    [Fact]
+    public void Vertex_Position_At_Offset_0()
+    {
+        Assert.Equal(0, Marshal.OffsetOf<Vertex>("Position"));
+    }
+
+    [Fact]
+    public void Vertex_Color_At_Offset_12()
+    {
+        Assert.Equal(12, Marshal.OffsetOf<Vertex>("Color"));
+    }
+
+    [Fact]
+    public void Vertex_Normal_At_Offset_24()
+    {
+        Assert.Equal(24, Marshal.OffsetOf<Vertex>("Normal"));
+    }
+}