fix: close polygon offset shape and handle nest template load failure

Shape.OffsetEntity computed joins between consecutive offset segments
but never joined the last segment back to the first, leaving the
closing corner with a straight line instead of a proper miter/arc.
Track the first entity and apply the same join logic after the loop.

Also wrap nest template loading in try-catch so a corrupt template
file doesn't crash the app on startup — falls back to default nest.

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

OpenNest.Core/Geometry/Shape.cs

@@ -463,6 +463,8 @@ namespace OpenNest.Geometry
             var offsetShape = new Shape();
             var definedShape = new ShapeProfile(this);
 
+            Entity firstEntity = null;
+            Entity firstOffsetEntity = null;
             Entity lastEntity = null;
             Entity lastOffsetEntity = null;
 
@@ -473,6 +475,12 @@ namespace OpenNest.Geometry
                 if (offsetEntity == null)
                     continue;
 
+                if (firstEntity == null)
+                {
+                    firstEntity = entity;
+                    firstOffsetEntity = offsetEntity;
+                }
+
                 switch (entity.Type)
                 {
                     case EntityType.Line:
@@ -482,12 +490,48 @@ namespace OpenNest.Geometry
 
                             if (lastOffsetEntity != null && lastOffsetEntity.Type == EntityType.Line)
                             {
-                                var lastLine = lastEntity as Line;
+                                JoinOffsetLines(
-                                var lastOffsetLine = lastOffsetEntity as Line;
+                                    (Line)lastEntity, (Line)lastOffsetEntity,
+                                    line, offsetLine,
+                                    distance, side, offsetShape);
+                            }
 
-                                if (lastLine == null || lastOffsetLine == null)
+                            offsetShape.Entities.Add(offsetLine);
-                                    continue;
+                            break;
+                        }
 
+                    default:
+                        offsetShape.Entities.Add(offsetEntity);
+                        break;
+                }
+
+                lastOffsetEntity = offsetEntity;
+                lastEntity = entity;
+            }
+
+            // Close the shape: join last offset entity back to first
+            if (lastOffsetEntity != null && firstOffsetEntity != null
+                && lastOffsetEntity != firstOffsetEntity
+                && lastOffsetEntity.Type == EntityType.Line
+                && firstOffsetEntity.Type == EntityType.Line)
+            {
+                JoinOffsetLines(
+                    (Line)lastEntity, (Line)lastOffsetEntity,
+                    (Line)firstEntity, (Line)firstOffsetEntity,
+                    distance, side, offsetShape);
+            }
+
+            foreach (var cutout in definedShape.Cutouts)
+                offsetShape.Entities.AddRange(((Shape)cutout.OffsetEntity(distance, side)).Entities);
+
+            return offsetShape;
+        }
+
+        private static void JoinOffsetLines(
+            Line lastLine, Line lastOffsetLine,
+            Line line, Line offsetLine,
+            double distance, OffsetSide side, Shape offsetShape)
+        {
             Vector intersection;
 
             if (Helper.Intersects(offsetLine, lastOffsetLine, out intersection))
@@ -509,25 +553,6 @@ namespace OpenNest.Geometry
             }
         }
 
-                            offsetShape.Entities.Add(offsetLine);
-                            break;
-                        }
-
-                    default:
-                        offsetShape.Entities.Add(offsetEntity);
-                        break;
-                }
-
-                lastOffsetEntity = offsetEntity;
-                lastEntity = entity;
-            }
-
-            foreach (var cutout in definedShape.Cutouts)
-                offsetShape.Entities.AddRange(((Shape)cutout.OffsetEntity(distance, side)).Entities);
-
-            return offsetShape;
-        }
-
         public override Entity OffsetEntity(double distance, Vector pt)
         {
             throw new NotImplementedException();

OpenNest/Forms/MainForm.cs

@@ -47,6 +47,17 @@ namespace OpenNest.Forms
             //    BestFitCache.CreateEvaluator = (drawing, spacing) => GpuEvaluatorFactory.Create(drawing, spacing);
         }
 
+        private Nest CreateDefaultNest()
+        {
+            var nest = new Nest();
+            nest.Units = Properties.Settings.Default.DefaultUnit;
+            nest.PlateDefaults.EdgeSpacing = new Spacing(1, 1, 1, 1);
+            nest.PlateDefaults.PartSpacing = 1;
+            nest.PlateDefaults.Size = new OpenNest.Geometry.Size(100, 100);
+            nest.PlateDefaults.Quadrant = 1;
+            return nest;
+        }
+
         private string GetNestName(DateTime date, int id)
         {
             var month = date.Month.ToString().PadLeft(2, '0');
@@ -325,18 +336,25 @@ namespace OpenNest.Forms
             Nest nest;
 
             if (File.Exists(Properties.Settings.Default.NestTemplatePath))
+            {
+                try
                 {
                     var reader = new NestReader(Properties.Settings.Default.NestTemplatePath);
                     nest = reader.Read();
                 }
+                catch (Exception ex)
+                {
+                    MessageBox.Show(
+                        $"Failed to load nest template:\n{ex.Message}\n\nA default nest will be created instead.",
+                        "Template Error",
+                        MessageBoxButtons.OK,
+                        MessageBoxIcon.Warning);
+                    nest = CreateDefaultNest();
+                }
+            }
             else
             {
-                nest = new Nest();
+                nest = CreateDefaultNest();
-                nest.Units = Properties.Settings.Default.DefaultUnit;
-                nest.PlateDefaults.EdgeSpacing = new Spacing(0, 0, 0, 0);
-                nest.PlateDefaults.PartSpacing = 0;
-                nest.PlateDefaults.Size = new OpenNest.Geometry.Size(100, 100);
-                nest.PlateDefaults.Quadrant = 1;
             }
 
             nest.DateCreated = DateTime.Now;