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/CadImporter.cs

@@ -42,7 +42,6 @@ namespace OpenNest.IO
                 Entities = dxf.Entities,
                 Bends = bends,
                 Bounds = dxf.Entities.GetBoundingBox(),
-                Document = dxf.Document,
                 SourcePath = path,
                 Name = options.Name ?? Path.GetFileNameWithoutExtension(path),
             };