feat: integrate NFP optimization into nest engines and fill UI

Add Compactor.Settle and AutoNester.Optimize post-passes to
NestEngineBase.Nest, StripNestEngine, and PlateView.FillWithProgress
so all fill paths benefit from geometry-aware compaction.

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

OpenNest.Engine/NestEngineBase.cs

@@ -1,4 +1,5 @@
 using OpenNest.Engine.Fill;
+using OpenNest.Engine.Nfp;
 using OpenNest.Geometry;
 using System;
 using System.Collections.Generic;
@@ -126,6 +127,12 @@ namespace OpenNest
                 }
             }
 
+            // Compact placed parts toward the origin to close gaps.
+            Compactor.Settle(allParts, Plate.WorkArea(), Plate.PartSpacing);
+
+            // NFP optimization pass — re-place parts using geometry-aware BLF.
+            allParts = AutoNester.Optimize(allParts, Plate);
+
             return allParts;
         }