feat: rebuild Vulkan renderer from scratch — pure P/Invoke triangle (Vulkan 1.3)

- Complete rewrite of Engine.Graphics.Vulkan with pure P/Invoke (no wrapper libs)
- Vulkan 1.3: dynamic rendering (vkCmdBeginRendering/vkCmdEndRendering),
  synchronization2 (vkQueueSubmit2, vkCmdPipelineBarrier2)
- Split types into VulkanHandles.cs, VulkanEnums.cs, VulkanStructs.cs
- Staging buffer → device-local vertex buffer pattern
- Correct swapchain semaphore indexing (per-image, not per-frame-in-flight)
- VK_EXT_debug_utils debug messenger with validation layer fallback
- Dynamic viewport/scissor (no pipeline recreation on resize)
- Simplified Program.cs to triangle-only rendering
- Removed old Silk.NET renderer, ImGui, PBR shaders, screenshot code
- Updated VULKAN_IMPLEMENTATION_PLAN.md with full architecture decisions

src/Engine.Graphics/MeshMath.cs

@@ -2,17 +2,18 @@ using System.Numerics;
 
 namespace Engine.Graphics;
 
+/// <summary>
+/// Basic mesh math utilities.
+/// </summary>
 public static class MeshMath
 {
     public static Vector3 ComputeFaceNormal(Vector3 a, Vector3 b, Vector3 c)
     {
-        var edge1 = b - a;
-        var edge2 = c - a;
-        var normal = Vector3.Cross(edge2, edge1);
-
-        if (normal.LengthSquared() < 1e-12f)
+        var ab = b - a;
+        var ac = c - a;
+        var cross = Vector3.Cross(ab, ac);
+        if (cross.LengthSquared() < 0.0000001f)
             return Vector3.UnitY;
-
-        return Vector3.Normalize(normal);
+        return Vector3.Normalize(cross);
     }
 }