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/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;