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.IO/CadImportResult.cs

@@ -1,5 +1,4 @@
 using System.Collections.Generic;
-using ACadSharp;
 using OpenNest.Bending;
 using OpenNest.Geometry;
 
@@ -30,12 +29,6 @@ namespace OpenNest.IO
         /// </summary>
         public Box Bounds { get; set; }
 
-        /// <summary>
-        /// Underlying CAD document for callers that need to run additional
-        /// analysis (e.g., MText extraction). May be null for stream imports.
-        /// </summary>
-        public CadDocument Document { get; set; }
-
         /// <summary>
         /// Absolute path to the source file.
         /// </summary>