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.Tests/IO/CadImporterTests.cs

@@ -39,6 +39,18 @@ namespace OpenNest.Tests.IO
             Assert.Equal("custom", result.Name);
         }
 
+        [Fact]
+        public void Import_WhenNamedDetectorDoesNotExist_ReturnsEmptyBends()
+        {
+            // Exercises the named-detector branch: when BendDetectorName doesn't
+            // match any registered detector, bends should be an empty list
+            // (not a crash, and no fall-through to auto-detect).
+            var result = CadImporter.Import(TestDxf,
+                new CadImportOptions { BendDetectorName = "__nonexistent__" });
+
+            Assert.Empty(result.Bends);
+        }
+
         [Fact]
         public void BuildDrawing_ProducesDrawingWithProgramAndMetadata()
         {