feat: ImGui debug overlay — FPS, camera position, entity count

- ImGui.NET 1.91.6.1 integrated into Engine.Graphics.Vulkan
- VulkanImGui.cs: font atlas upload (staging → VkImage), sampler,
  descriptor set (COMBINED_IMAGE_SAMPLER), separate pipeline with
  alpha blending, no depth write, dynamic scissor
- ImGui shaders: imgui.vert (ortho MVP push constant) + imgui.frag
  (font texture sampler)
- Dynamic vertex/index buffers (HOST_VISIBLE, grow on demand)
- IRenderer interface: BeginImGuiFrame/EndImGuiFrame default methods
- Program.cs: debug window with FPS, camera pos/target, entity count
- 0 critical validation errors, ~750 FPS with physics + ImGui

src/Engine.Graphics.Vulkan/VulkanStructs.cs

@@ -957,6 +957,57 @@ public unsafe struct VkImageCreateInfo
     public VkImageLayout initialLayout;
 }
 
+[StructLayout(LayoutKind.Sequential)]
+public unsafe struct VkImageSubresourceLayers
+{
+    public VkImageAspectFlags aspectMask;
+    public uint mipLevel;
+    public uint baseArrayLayer;
+    public uint layerCount;
+}
+
+[StructLayout(LayoutKind.Sequential)]
+public unsafe struct VkBufferImageCopyRegion
+{
+    public ulong bufferOffset;
+    public uint bufferRowLength;
+    public uint bufferImageHeight;
+    public VkImageSubresourceLayers imageSubresource;
+    public VkOffset3D imageOffset;
+    public VkExtent3D imageExtent;
+}
+
+[StructLayout(LayoutKind.Sequential)]
+public unsafe struct VkSamplerCreateInfo
+{
+    public VkStructureType sType;
+    public nint pNext;
+    public uint flags;
+    public VkFilter magFilter;
+    public VkFilter minFilter;
+    public VkSamplerMipmapMode mipmapMode;
+    public VkSamplerAddressMode addressModeU;
+    public VkSamplerAddressMode addressModeV;
+    public VkSamplerAddressMode addressModeW;
+    public float mipLodBias;
+    public VkBool32 anisotropyEnable;
+    public float maxAnisotropy;
+    public VkBool32 compareEnable;
+    public VkCompareOp compareOp;
+    public float minLod;
+    public float maxLod;
+    public int borderColor;
+    public VkBool32 unnormalizedCoordinates;
+}
+
+[StructLayout(LayoutKind.Sequential)]
+public unsafe struct VkDescriptorImageInfo
+{
+    public VkSampler sampler;
+    public VkImageView imageView;
+    public VkImageLayout imageLayout;
+}
+
 [StructLayout(LayoutKind.Sequential)]
 public unsafe struct VkPushConstantRange
 {