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/VulkanRenderContext.cs

@@ -2,8 +2,7 @@ using Engine.Core;
 
 namespace Engine.Graphics.Vulkan;
 
-internal sealed class VulkanRenderContext : IRenderContext
-{
+internal sealed class VulkanRenderContext : IRenderContext{
     private readonly VulkanContext _ctx;
     private readonly VulkanSwapchain _swapchain;
     private readonly IWindow _window;