feat(engine): add PairsFillStrategy adapter

Wraps PairFiller in an IFillStrategy so the pairs phase participates
in the pluggable pipeline. Stores BestFitResults in SharedState for
downstream strategies (Extents) to reuse.

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

OpenNest.Engine/Strategies/PairsFillStrategy.cs

@@ -0,0 +1,27 @@
+using System.Collections.Generic;
+using OpenNest.Engine.BestFit;
+
+namespace OpenNest
+{
+    public class PairsFillStrategy : IFillStrategy
+    {
+        public string Name => "Pairs";
+        public NestPhase Phase => NestPhase.Pairs;
+        public int Order => 100;
+
+        public List<Part> Fill(FillContext context)
+        {
+            var filler = new PairFiller(context.Plate.Size, context.Plate.PartSpacing);
+            var result = filler.Fill(context.Item, context.WorkArea,
+                context.PlateNumber, context.Token, context.Progress);
+
+            // Cache hit — PairFiller already called GetOrCompute internally.
+            var bestFits = BestFitCache.GetOrCompute(
+                context.Item.Drawing, context.Plate.Size.Length,
+                context.Plate.Size.Width, context.Plate.PartSpacing);
+            context.SharedState["BestFits"] = bestFits;
+
+            return result;
+        }
+    }
+}