feat: add material library resolution, assist gas support, and UI fixes

- Add MaterialLibraryResolver for Cincinnati post processor to resolve
  G89 library files from material/thickness/gas configuration
- Add Nest.AssistGas property with serialization support in nest format
- Add etch library support with separate gas configuration
- Fix CutOff tests to match AwayFromOrigin default cut direction
- Fix plate info label not updating after ResizePlateToFitParts
- Add cutoff and remnants toolbar button icons

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

OpenNest.IO/NestFormat.cs

@@ -23,6 +23,7 @@ namespace OpenNest.IO
             public string DateCreated { get; init; } = "";
             public string DateLastModified { get; init; } = "";
             public string Notes { get; init; } = "";
+            public string AssistGas { get; init; } = "";
             public PlateDefaultsDto PlateDefaults { get; init; } = new();
             public List<DrawingDto> Drawings { get; init; } = new();
             public List<PlateDto> Plates { get; init; } = new();