feat: pure Vulkan P/Invoke renderer — no wrappers, SDL3 window, SPIR-V shaders

- Engine.Graphics: restored interfaces (IRenderContext, IRenderer, RenderBackendFactory),
  loaders (ObjLoader, GltfLoader), ProceduralMesh, MeshMath, SceneSerializer
- Engine.Graphics.Vulkan: pure P/Invoke to libvulkan.so.1/vulkan-1.dll
  - VulkanNative: library loading, function pointer loading via vkGetInstanceProcAddr
  - Vk: static function cache for ~80 Vulkan functions, all delegate types
  - VulkanTypes: ~50 structs, ~20 enums matching Vulkan C headers
  - VulkanContext: instance, physical device, logical device, surface, memory types
  - VulkanSwapchain: swapchain, image views, depth image, render pass, framebuffers
  - VulkanPipeline: graphics pipeline, descriptor set layout, shader modules
  - VulkanBuffer: vertex/index/uniform buffers, staging, memory allocation
  - VulkanRenderer: command buffers, sync (semaphores/fences), render loop, 2 frames in flight
  - GLSL 450 shaders: PBR lighting (Fresnel, ACES, gamma), directional+point lights
  - Compiled to SPIR-V via @webgpu/glslang WASM
- CortexEngine.App: switched to vulkan backend, removed Raylib/OpenTK/ImGui refs
- Tests: all 66 pass (ObjLoader, MeshMath, ProceduralMesh, SceneSerializer, etc.)
- Camera tour: 16 poses, screenshots captured, clean shutdown

src/Engine.Graphics/MeshMath.cs

@@ -0,0 +1,18 @@
+using System.Numerics;
+
+namespace Engine.Graphics;
+
+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)
+            return Vector3.UnitY;
+
+        return Vector3.Normalize(normal);
+    }
+}