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.Core/Nest.cs

@@ -36,6 +36,8 @@ namespace OpenNest
 
         public string Notes { get; set; }
 
+        public string AssistGas { get; set; } = "";
+
         public Units Units { get; set; }
 
         public DateTime DateCreated { get; set; }