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/VulkanRenderer.cs

@@ -13,6 +13,7 @@ internal sealed unsafe class VulkanRenderer : IRenderer, Engine.Graphics.IScreen
     private readonly VulkanSwapchain _swapchain;
     private readonly VulkanPipeline _pipeline;
     private readonly VulkanFrameResources _frameResources;
+    internal readonly VulkanImGui? _imGui;
 
     private readonly Dictionary<ulong, (VulkanVertexBuffer vb, VulkanIndexBuffer ib, uint indexCount)> _meshCache = new();
 
@@ -37,6 +38,21 @@ internal sealed unsafe class VulkanRenderer : IRenderer, Engine.Graphics.IScreen
 
         _frameResources = new VulkanFrameResources(ctx.Device, ctx.GraphicsQueueFamilyIndex,
             swapchain.ImageCount, ctx, _pipeline.DescriptorSetLayout);
+
+        _imGui = new VulkanImGui(ctx, _frameResources.CommandPool, swapchain.Format, swapchain.DepthFormat);
+    }
+
+    internal VulkanImGui? ImGuiLayer => _imGui;
+
+    public void BeginImGuiFrame()
+    {
+        _imGui?.NewFrame();
+    }
+
+    public void EndImGuiFrame()
+    {
+        if (_imGui == null) return;
+        ImGuiNET.ImGui.Render();
     }
 
     public void RenderWorld(World world)
@@ -205,6 +221,8 @@ internal sealed unsafe class VulkanRenderer : IRenderer, Engine.Graphics.IScreen
             Vk.vkCmdDrawIndexed(cmd, dc.indexCount, 1, 0, 0, 0);
         }
 
+        _imGui?.Render(cmd, _swapchain.Extent.Width, _swapchain.Extent.Height);
+
         Vk.vkCmdEndRendering(cmd);
 
         TransitionImageLayout(cmd, _swapchain.Images[imageIndex],
@@ -389,6 +407,7 @@ internal sealed unsafe class VulkanRenderer : IRenderer, Engine.Graphics.IScreen
         }
         _meshCache.Clear();
 
+        _imGui?.Dispose();
         _frameResources?.Dispose();
         _pipeline?.Dispose();
     }