feat: AI screenshot capture for visual analysis

- Add ScreenshotCapture class to Engine.Graphics using Vulkan image readback.
- Integrate screenshot capture into MeshRenderer; request triggers readback on next frame.
- Add SixLabors.ImageSharp 3.1.11 for PNG encoding (patched for CVE-2025-54575).
- Add capture_screenshot AI command and MCP tool.
- Wire screenshot request into Program.cs with demo command.
- Expose swapchain surface format and image accessor for readback.
- Add Screenshots/ to .gitignore.
- Update CORTEX_ENGINE_ARCHITECTURE.md with MCP, Silk.NET.Vulkan, ImageSharp, and screenshot capture.

src/Engine.AI/Commands/CaptureScreenshotCommand.cs

@@ -0,0 +1,12 @@
+namespace Engine.AI.Commands;
+
+/// <summary>
+/// Request a screenshot of the current rendered frame for AI visual analysis.
+/// </summary>
+public sealed record CaptureScreenshotCommand : AiCommand
+{
+    /// <summary>
+    /// Output file path. If omitted, the engine chooses a default path.
+    /// </summary>
+    public string? OutputPath { get; init; }
+}