feat: use direction-specific engines in StripNestEngine

Height shrink now uses HorizontalRemnantEngine (minimizes Y-extent)
and width shrink uses VerticalRemnantEngine (minimizes X-extent).
IterativeShrinkFiller accepts an optional widthFillFunc so each
shrink axis can use a different fill engine.

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

OpenNest.Engine/Fill/IterativeShrinkFiller.cs

@@ -31,7 +31,8 @@ namespace OpenNest.Engine.Fill
             double spacing,
             CancellationToken token = default,
             IProgress<NestProgress> progress = null,
-            int plateNumber = 0)
+            int plateNumber = 0,
+            Func<NestItem, Box, List<Part>> widthFillFunc = null)
         {
             if (items == null || items.Count == 0)
                 return new IterativeShrinkResult();
@@ -72,6 +73,8 @@ namespace OpenNest.Engine.Fill
             // include them in progress reports.
             var placedSoFar = new List<Part>();
 
+            var wFillFunc = widthFillFunc ?? fillFunc;
+
             Func<NestItem, Box, List<Part>> shrinkWrapper = (ni, box) =>
             {
                 var target = ni.Quantity > 0 ? ni.Quantity : 0;
@@ -84,7 +87,7 @@ namespace OpenNest.Engine.Fill
                 Parallel.Invoke(
                     () => heightResult = ShrinkFiller.Shrink(fillFunc, ni, box, spacing, ShrinkAxis.Height, token,
                         targetCount: target, progress: progress, plateNumber: plateNumber, placedParts: placedSoFar),
-                    () => widthResult = ShrinkFiller.Shrink(fillFunc, ni, box, spacing, ShrinkAxis.Width, token,
+                    () => widthResult = ShrinkFiller.Shrink(wFillFunc, ni, box, spacing, ShrinkAxis.Width, token,
                         targetCount: target, progress: progress, plateNumber: plateNumber, placedParts: placedSoFar)
                 );
 

OpenNest.Engine/StripNestEngine.cs

@@ -77,17 +77,23 @@ namespace OpenNest
             // Phase 1: Iterative shrink-fill for multi-quantity items.
             if (fillItems.Count > 0)
             {
-                // Pass progress through so the UI shows intermediate results
+                // Use direction-specific engines: height shrink benefits from
-                // during the initial BestFitCache computation and fill phases.
+                // minimizing Y-extent, width shrink from minimizing X-extent.
-                Func<NestItem, Box, List<Part>> fillFunc = (ni, b) =>
+                Func<NestItem, Box, List<Part>> heightFillFunc = (ni, b) =>
                 {
-                    var inner = new DefaultNestEngine(Plate);
+                    var inner = new HorizontalRemnantEngine(Plate);
+                    return inner.Fill(ni, b, progress, token);
+                };
+
+                Func<NestItem, Box, List<Part>> widthFillFunc = (ni, b) =>
+                {
+                    var inner = new VerticalRemnantEngine(Plate);
                     return inner.Fill(ni, b, progress, token);
                 };
 
                 var shrinkResult = IterativeShrinkFiller.Fill(
-                    fillItems, workArea, fillFunc, Plate.PartSpacing, token,
+                    fillItems, workArea, heightFillFunc, Plate.PartSpacing, token,
-                    progress, PlateNumber);
+                    progress, PlateNumber, widthFillFunc);
 
                 allParts.AddRange(shrinkResult.Parts);