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/AiCommandProcessor.cs

@@ -16,13 +16,15 @@ public sealed class AiCommandProcessor
 {
     private readonly World _world;
     private readonly Func<string, Mesh> _modelLoader;
+    private readonly Action<string> _requestScreenshot;
 
     public JsonSerializerOptions JsonOptions { get; }
 
-    public AiCommandProcessor(World world, Func<string, Mesh> modelLoader)
+    public AiCommandProcessor(World world, Func<string, Mesh> modelLoader, Action<string> requestScreenshot)
     {
         _world = world;
         _modelLoader = modelLoader;
+        _requestScreenshot = requestScreenshot;
         JsonOptions = new JsonSerializerOptions
         {
             PropertyNameCaseInsensitive = true,
@@ -52,6 +54,7 @@ public sealed class AiCommandProcessor
                 SetTransformCommand c => SetTransform(c),
                 DeleteEntityCommand c => DeleteEntity(c),
                 ListEntitiesCommand => ListEntities(),
+                CaptureScreenshotCommand c => CaptureScreenshot(c),
                 _ => AiCommandResult.Error($"Unknown command type: {command.Type}")
             };
         }
@@ -122,4 +125,11 @@ public sealed class AiCommandProcessor
         var json = JsonSerializer.Serialize(names, JsonOptions);
         return AiCommandResult.Ok(json);
     }
+
+    private AiCommandResult CaptureScreenshot(CaptureScreenshotCommand command)
+    {
+        var path = command.OutputPath ?? $"screenshot_{DateTime.UtcNow:yyyyMMdd_HHmmss_fff}.png";
+        _requestScreenshot(path);
+        return AiCommandResult.Ok($"Screenshot requested: {path}");
+    }
 }

src/Engine.AI/Commands/AiCommand.cs

@@ -11,6 +11,7 @@ namespace Engine.AI.Commands;
 [JsonDerivedType(typeof(SetTransformCommand), "set_transform")]
 [JsonDerivedType(typeof(DeleteEntityCommand), "delete_entity")]
 [JsonDerivedType(typeof(ListEntitiesCommand), "list_entities")]
+[JsonDerivedType(typeof(CaptureScreenshotCommand), "capture_screenshot")]
 public abstract record AiCommand
 {
     public string Type => GetType().Name.Replace("Command", "").ToLowerInvariant();

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; }
+}

src/Engine.AI/Mcp/EngineMcpTools.cs

@@ -70,6 +70,13 @@ public sealed class EngineMcpTools
         return EnqueueAndReturnMessage(cmd);
     }
 
+    [McpServerTool, Description("Capture a screenshot of the current rendered frame and save it to disk.")]
+    public Task<string> CaptureScreenshot([Description("Optional output file path (default: screenshot_<timestamp>.png)")] string? outputPath = null)
+    {
+        var cmd = new CaptureScreenshotCommand { OutputPath = outputPath };
+        return EnqueueAndReturnMessage(cmd);
+    }
+
     private async Task<string> EnqueueAndReturnMessage(AiCommand command)
     {
         var result = await _queue.EnqueueAsync(command).ConfigureAwait(false);