feat(cincinnati): seed material library defaults and add selector dropdown

Adds the full Cincinnati material/etch library list as the committed
default config (seeded into Posts/ on build only when no runtime config
exists), plus a Selected Library override in the PropertyGrid backed by
a TypeConverter that populates from MaterialLibraries. MainForm calls
the new IPostProcessorNestAware hook before showing the config so the
dropdown opens preselected to the best match by nest material and
nearest thickness.

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

OpenNest.Posts.Cincinnati/MaterialLibraryResolver.cs

@@ -10,15 +10,20 @@ public sealed class MaterialLibraryResolver
 
     private readonly List<MaterialLibraryEntry> _materialLibraries;
     private readonly List<EtchLibraryEntry> _etchLibraries;
+    private readonly string _selectedLibrary;
 
     public MaterialLibraryResolver(CincinnatiPostConfig config)
     {
         _materialLibraries = config.MaterialLibraries ?? new List<MaterialLibraryEntry>();
         _etchLibraries = config.EtchLibraries ?? new List<EtchLibraryEntry>();
+        _selectedLibrary = config.SelectedLibrary ?? "";
     }
 
     public string ResolveCutLibrary(string materialName, double thickness, string gas)
     {
+        if (!string.IsNullOrEmpty(_selectedLibrary))
+            return EnsureLibExtension(_selectedLibrary);
+
         var entry = _materialLibraries.FirstOrDefault(e =>
             string.Equals(e.Material, materialName, StringComparison.OrdinalIgnoreCase) &&
             System.Math.Abs(e.Thickness - thickness) <= ThicknessTolerance &&