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/IRenderer.cs

@@ -13,4 +13,16 @@ public interface IRenderer : IDisposable
     void RequestScreenshot(string path);
     bool IsScreenshotRequested { get; }
     IScreenshotProvider ScreenshotProvider { get; }
+
+    /// <summary>
+    /// Called before RenderWorld to begin a new ImGui frame.
+    /// Null if the backend doesn't support ImGui.
+    /// </summary>
+    void BeginImGuiFrame() { }
+
+    /// <summary>
+    /// Called after RenderWorld to render ImGui draw data.
+    /// Null if the backend doesn't support ImGui.
+    /// </summary>
+    void EndImGuiFrame() { }
 }