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();

OpenNest.IO/NestReader.cs

@@ -160,6 +160,7 @@ namespace OpenNest.IO
             nest.DateCreated = DateTime.Parse(dto.DateCreated);
             nest.DateLastModified = DateTime.Parse(dto.DateLastModified);
             nest.Notes = dto.Notes;
+            nest.AssistGas = dto.AssistGas ?? "";
 
             // Plate defaults
             var pd = dto.PlateDefaults;

OpenNest.IO/NestWriter.cs

@@ -77,6 +77,7 @@ namespace OpenNest.IO
                 DateCreated = nest.DateCreated.ToString("o"),
                 DateLastModified = nest.DateLastModified.ToString("o"),
                 Notes = nest.Notes ?? "",
+                AssistGas = nest.AssistGas ?? "",
                 PlateDefaults = BuildPlateDefaultsDto(),
                 Drawings = BuildDrawingDtos(),
                 Plates = BuildPlateDtos()