chore(cad-importer): remove dead code and cover named detector branch

- Drop CadImportResult.Document: no caller reads it after the
  migrations (BendDetectorRegistry runs inside CadImporter.Import
  itself, and downstream callers only consume the entity/bend data).
- Drop dead CadConverterForm.GetNextColor() helper: zero callers
  since GetDrawings stopped needing it.
- Drop stale 'using OpenNest.Properties;' and unused 'newItems'
  local in OnSplitClicked.
- Add Import_WhenNamedDetectorDoesNotExist_ReturnsEmptyBends to
  cover the previously untested named-detector branch in
  CadImporter.Import.

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

OpenNest/Forms/CadConverterForm.cs

@@ -5,7 +5,6 @@ using OpenNest.Converters;
 using OpenNest.Geometry;
 using OpenNest.IO;
 using OpenNest.IO.Bending;
-using OpenNest.Properties;
 using System;
 using System.Collections.Generic;
 using System.Drawing;
@@ -356,7 +355,6 @@ namespace OpenNest.Forms
                 : Path.GetTempPath();
 
             var index = fileList.SelectedIndex;
-            var newItems = new List<string>();
 
             var splitWriter = new SplitDxfWriter();
             var splitItems = new List<FileListItem>();
@@ -369,7 +367,6 @@ namespace OpenNest.Forms
                 var splitPath = GetUniquePath(Path.Combine(writableDir, splitName));
 
                 splitWriter.Write(splitPath, splitDrawing);
-                newItems.Add(splitPath);
 
                 // Re-import geometry but keep bends from the split drawing
                 var result = Dxf.Import(splitPath);
@@ -750,9 +747,6 @@ namespace OpenNest.Forms
                 item.SuppressedEntityIds = null;
         }
 
-
-        private static Color GetNextColor() => Drawing.GetNextColor();
-
         private static bool IsDirectoryWritable(string path)
         {
             try