feat(engine): add ML feature extraction and brute-force runner

Add FeatureExtractor for computing geometric part features (convexity,
aspect ratio, circularity, bitmask) and BruteForceRunner for generating
training data by running the fill engine and recording results.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/ML/BruteForceRunner.cs

@@ -0,0 +1,53 @@
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using OpenNest.Geometry;
+
+namespace OpenNest.Engine.ML
+{
+    public class BruteForceResult
+    {
+        public int PartCount { get; set; }
+        public double Utilization { get; set; }
+        public long TimeMs { get; set; }
+        public string LayoutData { get; set; }
+        public List<Part> PlacedParts { get; set; }
+    }
+
+    public static class BruteForceRunner
+    {
+        public static BruteForceResult Run(Drawing drawing, Plate plate)
+        {
+            var engine = new NestEngine(plate);
+            var item = new NestItem { Drawing = drawing };
+
+            var sw = Stopwatch.StartNew();
+            var parts = engine.Fill(item, plate.WorkArea(), null, System.Threading.CancellationToken.None);
+            sw.Stop();
+
+            if (parts == null || parts.Count == 0)
+                return null;
+
+            return new BruteForceResult
+            {
+                PartCount = parts.Count,
+                Utilization = CalculateUtilization(parts, plate.Area()),
+                TimeMs = sw.ElapsedMilliseconds,
+                LayoutData = SerializeLayout(parts),
+                PlacedParts = parts
+            };
+        }
+
+        private static string SerializeLayout(List<Part> parts)
+        {
+            var data = parts.Select(p => new { X = p.Location.X, Y = p.Location.Y, R = p.Rotation }).ToList();
+            return System.Text.Json.JsonSerializer.Serialize(data);
+        }
+
+        private static double CalculateUtilization(List<Part> parts, double plateArea)
+        {
+            if (plateArea <= 0) return 0;
+            return parts.Sum(p => p.BaseDrawing.Area) / plateArea;
+        }
+    }
+}