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/VulkanContext.cs

@@ -5,7 +5,7 @@ using SDL;
 
 namespace Engine.Graphics.Vulkan;
 
-internal sealed unsafe class VulkanContext : IDisposable
+public sealed unsafe class VulkanContext : IDisposable
 {
     public VkInstance Instance;
     public VkPhysicalDevice PhysicalDevice;