fix(ui): center parts within the plate work area

Center Parts centred the parts on the whole plate, ignoring edge
spacing, so a wider clamp-side margin left the parts closer to the
clamps than the opposite edge. It now centres them in Plate.WorkArea()
via the new Plate.OffsetToCenterPartsInWorkArea, so each side keeps
the same extra room beyond its edge spacing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Core/Plate.cs

@@ -710,6 +710,14 @@ namespace OpenNest
             return box;
         }
 
+        /// <summary>
+        /// The offset that centers the parts' combined bounds in the <see cref="WorkArea"/>,
+        /// so uneven edge spacing (such as a wider clamp-side margin) shifts them with it.
+        /// Zero when the plate has no parts.
+        /// </summary>
+        public Vector OffsetToCenterPartsInWorkArea() =>
+            Parts.Count == 0 ? new Vector() : WorkArea().Center - Parts.GetBoundingBox().Center;
+
         /// <summary>
         /// Automatically sizes the plate to fit the parts.
         /// </summary>

OpenNest.Tests/PlateCenterPartsTests.cs

@@ -0,0 +1,50 @@
+using OpenNest.CNC;
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests;
+
+public class PlateCenterPartsTests
+{
+    private static Part MakeSquare(Vector location)
+    {
+        var pgm = new Program();
+        pgm.Codes.Add(new RapidMove(new Vector(0, 0)));
+        pgm.Codes.Add(new LinearMove(new Vector(10, 0)));
+        pgm.Codes.Add(new LinearMove(new Vector(10, 10)));
+        pgm.Codes.Add(new LinearMove(new Vector(0, 10)));
+        pgm.Codes.Add(new LinearMove(new Vector(0, 0)));
+        return new Part(new Drawing("square", pgm), location);
+    }
+
+    [Fact]
+    public void CentersPartsInWorkAreaNotPlate()
+    {
+        var plate = new Plate(100, 100)
+        {
+            // Wider clamp-side margin at the bottom.
+            EdgeSpacing = new Spacing(left: 0.25, bottom: 0.75, right: 0.25, top: 0.25),
+        };
+        var part = MakeSquare(new Vector(5, 5));
+        plate.Parts.Add(part);
+
+        part.Offset(plate.OffsetToCenterPartsInWorkArea());
+
+        var box = part.BoundingBox;
+        // Work area is X 0.25..99.75, Y 0.75..99.75, so the centre sits 0.25 above the plate's.
+        Assert.Equal(50, box.Center.X, 6);
+        Assert.Equal(50.25, box.Center.Y, 6);
+        Assert.Equal(box.Left - 0.25, 99.75 - box.Right, 6);
+        Assert.Equal(box.Bottom - 0.75, 99.75 - box.Top, 6);
+    }
+
+    [Fact]
+    public void EmptyPlateNeedsNoOffset()
+    {
+        var plate = new Plate(100, 100) { EdgeSpacing = new Spacing(0.25, 0.75, 0.25, 0.25) };
+
+        var offset = plate.OffsetToCenterPartsInWorkArea();
+
+        Assert.Equal(0, offset.X);
+        Assert.Equal(0, offset.Y);
+    }
+}

OpenNest/Forms/MainForm.Tools.cs

@@ -229,10 +229,7 @@ namespace OpenNest.Forms
             if (view?.Plate == null || view.Parts.Count == 0)
                 return;
 
-            var plateCenter = view.Plate.BoundingBox(false).Center;
-            var partsCenter = view.Parts.GetBoundingBox().Center;
-
-            var offset = plateCenter - partsCenter;
+            var offset = view.Plate.OffsetToCenterPartsInWorkArea();
 
             foreach (var part in view.Parts)
                 part.Offset(offset);