feat: add DrawOffset toggle to PlateView

Shows the offset geometry (PartSpacing buffer) around each part as a
semi-transparent red outline. Toggle via PlateView.DrawOffset property.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest/Controls/PlateView.cs

@@ -9,6 +9,7 @@ using System.Windows.Forms;
 using OpenNest.Actions;
 using OpenNest.CNC;
 using OpenNest.Collections;
+using OpenNest.Converters;
 using OpenNest.Geometry;
 using OpenNest.Math;
 using Action = OpenNest.Actions.Action;
@@ -73,6 +74,7 @@ namespace OpenNest.Controls
             DrawOrigin = true;
             DrawRapid = false;
             DrawBounds = true;
+            DrawOffset = false;
             FillParts = true;
             SetAction(typeof(ActionSelect));
 
@@ -93,6 +95,8 @@ namespace OpenNest.Controls
 
         public bool DrawBounds { get; set; }
 
+        public bool DrawOffset { get; set; }
+
         public bool FillParts { get; set; }
 
         public double RotateIncrementAngle { get; set; }
@@ -425,6 +429,9 @@ namespace OpenNest.Controls
                 part.Draw(g, (i + 1).ToString());
             }
 
+            if (DrawOffset && Plate.PartSpacing > 0)
+                DrawOffsetGeometry(g);
+
             if (DrawBounds)
             {
                 var bounds = SelectedParts.Select(p => p.BasePart).ToList().GetBoundingBox();
@@ -435,6 +442,34 @@ namespace OpenNest.Controls
                 DrawRapids(g);
         }
 
+        private void DrawOffsetGeometry(Graphics g)
+        {
+            using (var offsetPen = new Pen(Color.FromArgb(120, 255, 100, 100)))
+            {
+                for (int i = 0; i < parts.Count; i++)
+                {
+                    var part = parts[i].BasePart;
+                    var entities = ConvertProgram.ToGeometry(part.Program);
+                    var shapes = Helper.GetShapes(entities.Where(e => e.Layer != SpecialLayers.Rapid));
+
+                    foreach (var shape in shapes)
+                    {
+                        var offsetEntity = shape.OffsetEntity(Plate.PartSpacing, OffsetSide.Left) as Shape;
+
+                        if (offsetEntity == null)
+                            continue;
+
+                        offsetEntity.Offset(part.Location);
+
+                        var path = GraphicsHelper.GetGraphicsPath(offsetEntity);
+                        path.Transform(Matrix);
+                        g.DrawPath(offsetPen, path);
+                        path.Dispose();
+                    }
+                }
+            }
+        }
+
         private void DrawRapids(Graphics g)
         {
             var pos = new Vector(0, 0);