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.Vulkan/Shaders/vertex.vert

@@ -0,0 +1,34 @@
+#version 450
+
+layout(location = 0) in vec3 inPosition;
+layout(location = 1) in vec3 inColor;
+layout(location = 2) in vec3 inNormal;
+
+layout(row_major, set = 0, binding = 0) uniform FrameUBO {
+    vec3 cameraPosition;
+    uint lightCount;
+    vec3 ambientColor;
+    float pad0;
+    vec4 lightData[16];
+} frame;
+
+layout(push_constant) uniform PushConstants {
+    mat4 mvp;
+    mat4 model;
+    vec4 material;
+} pc;
+
+layout(location = 0) out vec3 fragColor;
+layout(location = 1) out vec3 fragNormal;
+layout(location = 2) out vec3 fragWorldPos;
+layout(location = 3) out vec3 fragViewDir;
+
+void main() {
+    vec4 worldPos = pc.model * vec4(inPosition, 1.0);
+    gl_Position = pc.mvp * vec4(inPosition, 1.0);
+
+    fragColor = inColor;
+    fragNormal = normalize(mat3(pc.model) * inNormal);
+    fragWorldPos = worldPos.xyz;
+    fragViewDir = normalize(frame.cameraPosition - worldPos.xyz);
+}