fix: prevent etch line layers from defaulting to layer 0 after split

DxfImporter now filters ETCH entities (like BEND) since etch marks are
generated from bends during export, not cut geometry. GeometryOptimizer
no longer merges lines/arcs across different layers and preserves layer
and color on merged entities. EntityView draws etch marks directly from
the Bends list so they remain visible without relying on imported ETCH
entities.

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

OpenNest/Controls/EntityView.cs

@@ -84,6 +84,8 @@ namespace OpenNest.Controls
                 DrawEntity(e.Graphics, entity, pen);
             }
 
+            DrawEtchMarks(e.Graphics);
+
 #if DRAW_OFFSET
 
             var offsetShape = new Shape();
@@ -185,6 +187,46 @@ namespace OpenNest.Controls
         private static bool IsEtchLayer(Layer layer) =>
             string.Equals(layer?.Name, "ETCH", System.StringComparison.OrdinalIgnoreCase);
 
+        private void DrawEtchMarks(Graphics g)
+        {
+            if (Bends == null || Bends.Count == 0)
+                return;
+
+            using var etchPen = new Pen(Color.Green, 1.5f);
+            var etchLength = 1.0;
+
+            foreach (var bend in Bends)
+            {
+                if (bend.Direction != BendDirection.Up)
+                    continue;
+
+                var start = bend.StartPoint;
+                var end = bend.EndPoint;
+                var length = bend.Length;
+
+                if (length < etchLength * 3.0)
+                {
+                    var pt1 = PointWorldToGraph(start);
+                    var pt2 = PointWorldToGraph(end);
+                    g.DrawLine(etchPen, pt1, pt2);
+                }
+                else
+                {
+                    var angle = start.AngleTo(end);
+                    var dx = System.Math.Cos(angle) * etchLength;
+                    var dy = System.Math.Sin(angle) * etchLength;
+
+                    var s1 = PointWorldToGraph(start);
+                    var e1 = PointWorldToGraph(new Vector(start.X + dx, start.Y + dy));
+                    g.DrawLine(etchPen, s1, e1);
+
+                    var s2 = PointWorldToGraph(end);
+                    var e2 = PointWorldToGraph(new Vector(end.X - dx, end.Y - dy));
+                    g.DrawLine(etchPen, s2, e2);
+                }
+            }
+        }
+
         private void DrawBendLines(Graphics g)
         {
             if (Bends == null || Bends.Count == 0)