feat: implement color and line type exclusion filters on CadConverterForm

Add per-entity IsVisible flag and wire up the Colors and Line Types
checkedlistboxes to filter entities by exclusion — checking an item
hides matching entities from the preview and from drawing export.

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

OpenNest.Core/Geometry/Entity.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Drawing;
 using OpenNest.Math;
 
 namespace OpenNest.Geometry
@@ -13,6 +14,21 @@ namespace OpenNest.Geometry
             boundingBox = new Box();
         }
 
+        /// <summary>
+        /// Entity color (resolved from DXF ByLayer/ByBlock to actual color).
+        /// </summary>
+        public Color Color { get; set; }
+
+        /// <summary>
+        /// Entity linetype name (e.g. "Continuous", "Center", "Dashed").
+        /// </summary>
+        public string LineTypeName { get; set; }
+
+        /// <summary>
+        /// Whether this entity is visible (used for color/linetype filtering).
+        /// </summary>
+        public bool IsVisible { get; set; } = true;
+
         /// <summary>
         /// Smallest box that contains the entity.
         /// </summary>