fix: prevent RemnantFiller interleaving and PairFiller recursion

RemnantFiller: add placed parts as a single envelope obstacle instead
of individual bounding boxes to prevent the next drawing from filling
into inter-row gaps. Remove the topmost bounding-box part to create a
clean rectangular boundary.

PairsFillStrategy: guard against recursive invocation — remnant fills
within PairFiller create a new engine that runs the full pipeline,
which would invoke PairsFillStrategy again causing deep recursion.

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

OpenNest.Engine/Strategies/PairsFillStrategy.cs

@@ -1,25 +1,42 @@
 using OpenNest.Engine.Fill;
 using System.Collections.Generic;
+using System.Threading;
 
 namespace OpenNest.Engine.Strategies
 {
     public class PairsFillStrategy : IFillStrategy
     {
+        private static readonly AsyncLocal<bool> active = new();
+
         public string Name => "Pairs";
         public NestPhase Phase => NestPhase.Pairs;
         public int Order => 100;
 
         public List<Part> Fill(FillContext context)
         {
-            var comparer = context.Policy?.Comparer;
-            var dedup = GridDedup.GetOrCreate(context.SharedState);
-            var filler = new PairFiller(context.Plate, comparer, dedup);
-            var result = filler.Fill(context.Item, context.WorkArea,
-                context.PlateNumber, context.Token, context.Progress);
+            // Prevent recursive PairFiller — remnant fills within PairFiller
+            // create a new engine that runs the full pipeline, which would
+            // invoke PairsFillStrategy again, causing deep recursion.
+            if (active.Value)
+                return null;
 
-            context.SharedState["BestFits"] = result.BestFits;
+            active.Value = true;
+            try
+            {
+                var comparer = context.Policy?.Comparer;
+                var dedup = GridDedup.GetOrCreate(context.SharedState);
+                var filler = new PairFiller(context.Plate, comparer, dedup);
+                var result = filler.Fill(context.Item, context.WorkArea,
+                    context.PlateNumber, context.Token, context.Progress);
 
-            return result.Parts;
+                context.SharedState["BestFits"] = result.BestFits;
+
+                return result.Parts;
+            }
+            finally
+            {
+                active.Value = false;
+            }
         }
     }
 }