feat(engine): add ExtentsFillStrategy adapter

Wraps FillExtents in an IFillStrategy, trying both bestRotation and
bestRotation+90° angles and picking the better result. Reads
BestFits from SharedState (populated by PairsFillStrategy) to allow
FillExtents to search the best-fit cache for improved pair geometry.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

OpenNest.Engine/Strategies/ExtentsFillStrategy.cs

@@ -0,0 +1,49 @@
+using System.Collections.Generic;
+using OpenNest.Engine.BestFit;
+using OpenNest.Math;
+
+namespace OpenNest
+{
+    public class ExtentsFillStrategy : IFillStrategy
+    {
+        public string Name => "Extents";
+        public NestPhase Phase => NestPhase.Extents;
+        public int Order => 300;
+
+        public List<Part> Fill(FillContext context)
+        {
+            var filler = new FillExtents(context.WorkArea, context.Plate.PartSpacing);
+
+            var bestRotation = context.SharedState.TryGetValue("BestRotation", out var rot)
+                ? (double)rot
+                : RotationAnalysis.FindBestRotation(context.Item);
+
+            var angles = new[] { bestRotation, bestRotation + Angle.HalfPI };
+
+            var bestFits = context.SharedState.TryGetValue("BestFits", out var cached)
+                ? (List<BestFitResult>)cached
+                : null;
+
+            List<Part> best = null;
+            var bestScore = default(FillScore);
+
+            foreach (var angle in angles)
+            {
+                context.Token.ThrowIfCancellationRequested();
+                var result = filler.Fill(context.Item.Drawing, angle,
+                    context.PlateNumber, context.Token, context.Progress, bestFits);
+                if (result != null && result.Count > 0)
+                {
+                    var score = FillScore.Compute(result, context.WorkArea);
+                    if (best == null || score > bestScore)
+                    {
+                        best = result;
+                        bestScore = score;
+                    }
+                }
+            }
+
+            return best ?? new List<Part>();
+        }
+    }
+}