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/Controls/EntityView.cs

@@ -103,7 +103,7 @@ namespace OpenNest.Controls
 
         private void DrawEntity(Graphics g, Entity e, Pen pen)
         {
-            if (!e.Layer.IsVisible)
+            if (!e.Layer.IsVisible || !e.IsVisible)
                 return;
 
             switch (e.Type)