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/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>