fix(engine): shrink the requested axis in ShrinkFiller estimates

ShrinkAxis.Length must shrink the Y extent and ShrinkAxis.Width the X
extent to agree with MeasureDimension/TrimToCount; Box's constructor
takes the X extent first. The estimate previously shrank the wrong
dimension and returned a mis-ordered box. Add a parameterized regression
test over both axes and both translated-remnant orientations.

OpenNest.Engine.Tests/Fill/ShrinkWorkAreaTests.cs

@@ -0,0 +1,40 @@
+using OpenNest.Engine.Fill;
+using OpenNest.Engine.Tests.Jobs;
+using OpenNest.Geometry;
+
+namespace OpenNest.Engine.Tests.Fill;
+
+public class ShrinkWorkAreaTests
+{
+    [Theory]
+    [InlineData(ShrinkAxis.Length, 30, 60)]
+    [InlineData(ShrinkAxis.Length, 60, 30)]
+    [InlineData(ShrinkAxis.Width, 30, 60)]
+    [InlineData(ShrinkAxis.Width, 60, 30)]
+    public void EstimateStartBox_ShrinksOnlyRequestedAxis_WithinTranslatedRectangularRemnant(
+        ShrinkAxis axis, double length, double width)
+    {
+        var item = new NestItem
+        {
+            Drawing = new Drawing("rectangle", TestDrawingFactory.Rectangle(4, 3)),
+            Quantity = 2,
+        };
+        var box = new Box(80, 7, length, width);
+
+        var estimate = ShrinkFiller.EstimateStartBox(item, box, 0.3, axis, 2);
+
+        Assert.Equal(box.Location, estimate.Location);
+        Assert.True(box.Contains(estimate), "A shrink estimate must never expand beyond its input remnant.");
+        // ShrinkAxis follows the fill/trim direction: Length trims top, Width trims right.
+        if (axis == ShrinkAxis.Length)
+        {
+            Assert.Equal(box.Length, estimate.Length);
+            Assert.InRange(estimate.Width, double.Epsilon, box.Width - double.Epsilon);
+        }
+        else
+        {
+            Assert.Equal(box.Width, estimate.Width);
+            Assert.InRange(estimate.Length, double.Epsilon, box.Length - double.Epsilon);
+        }
+    }
+}

OpenNest.Engine/Fill/ShrinkFiller.cs

@@ -120,7 +120,9 @@ namespace OpenNest.Engine.Fill
             if (bbox.Width <= 0 || bbox.Length <= 0)
                 return box;
 
-            var maxDim = axis == ShrinkAxis.Length ? box.Length : box.Width;
+            // Match MeasureDimension/TrimToCount: Length shrinks Y, Width shrinks X.
+            // Box's constructor takes X extent (Length), then Y extent (Width).
+            var maxDim = axis == ShrinkAxis.Length ? box.Width : box.Length;
 
             // Use FillBestFit for a fast, accurate rectangle count on the full box.
             var bin = new Bin { Size = new Size(box.Width, box.Length) };
@@ -144,8 +146,8 @@ namespace OpenNest.Engine.Fill
                 return box;
 
             return axis == ShrinkAxis.Length
-                ? new Box(box.X, box.Y, box.Width, estimate)
+                ? new Box(box.X, box.Y, box.Length, estimate)
-                : new Box(box.X, box.Y, estimate, box.Length);
+                : new Box(box.X, box.Y, estimate, box.Width);
         }
 
         private static double MeasureDimension(List<Part> parts, Box box, ShrinkAxis axis)