feat: ImGui integration + real screenshot capture via Vulkan

ImGui:
- ImGui.NET 1.91.6.1 NuGet package
- VulkanImGui: font texture upload, ImGui pipeline (blending, no depth), vertex/index buffer streaming
- GLSL 450 ImGui shaders (vert: scale/translate push constants, frag: font texture sampler)
- Compiled to SPIR-V via @webgpu/glslang
- Debug overlay: FPS, camera position, entity count, light list
- 1600+ FPS with ImGui active

Screenshot:
- vkCmdCopyImageToBuffer embedded in main command buffer
- Layout transition PresentSrcKHR→TransferSrcOptimal→PresentSrcKHR
- Deferred read: staging buffer read on next frame after fence
- BMP format written directly to FileStream (row by row)
- All 16 camera tour screenshots captured (1280x720x32bpp)

All 66 tests pass.

src/Engine.Graphics.Vulkan/VulkanRenderer.cs

@@ -7,11 +7,12 @@ using Flecs.NET.Core;
 
 namespace Engine.Graphics.Vulkan;
 
-internal sealed unsafe class VulkanRenderer : IRenderer, IScreenshotProvider
+public sealed unsafe class VulkanRenderer : IRenderer, IScreenshotProvider
 {
-    private readonly VulkanContext _ctx;
-    private readonly VulkanSwapchain _swapchain;
+    public readonly VulkanContext _ctx;
+    public readonly VulkanSwapchain _swapchain;
     private readonly VulkanPipeline _pipeline;
+    public VulkanImGui? ImGuiLayer;
 
     private VkCommandPool _commandPool;
     private VkCommandBuffer[] _commandBuffers = Array.Empty<VkCommandBuffer>();
@@ -536,6 +537,11 @@ internal sealed unsafe class VulkanRenderer : IRenderer, IScreenshotProvider
             }
         });
 
+        if (ImGuiLayer != null)
+        {
+            ImGuiLayer.Render(cmd);
+        }
+
         Vk.vkCmdEndRenderPass(cmd);
 
         if (_screenshotRequested)