fix: only remove bend-generated etch entities, preserve user etch lines

UpdateEtchEntities was removing all entities on the ETCH layer, which
also deleted user-added etch marks like part numbers. Now tags generated
bend etch lines with a BendEtch tag and filters on that instead.

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

OpenNest.Core/Bending/Bend.cs

@@ -14,6 +14,7 @@ namespace OpenNest.Bending
         };
 
         private const double DefaultEtchLength = 1.0;
+        private const string BendEtchTag = "BendEtch";
 
         public Vector StartPoint { get; set; }
         public Vector EndPoint { get; set; }
@@ -73,7 +74,7 @@ namespace OpenNest.Bending
         /// </summary>
         public static void UpdateEtchEntities(List<Entity> entities, List<Bend> bends)
         {
-            entities.RemoveAll(e => e.Layer == EtchLayer);
+            entities.RemoveAll(e => e.Tag == BendEtchTag);
             if (bends == null) return;
 
             foreach (var bend in bends)
@@ -82,7 +83,7 @@ namespace OpenNest.Bending
 
         private static Line CreateEtchLine(Vector start, Vector end)
         {
-            return new Line(start, end) { Layer = EtchLayer, Color = Color.Green };
+            return new Line(start, end) { Layer = EtchLayer, Color = Color.Green, Tag = BendEtchTag };
         }
 
         public override string ToString()

OpenNest.Core/Geometry/Entity.cs

@@ -29,6 +29,11 @@ namespace OpenNest.Geometry
         /// </summary>
         public bool IsVisible { get; set; } = true;
 
+        /// <summary>
+        /// Optional tag for identifying generated entities (e.g. bend etch marks).
+        /// </summary>
+        public string Tag { get; set; }
+
         /// <summary>
         /// Smallest box that contains the entity.
         /// </summary>