Add sub-asset import and mesh asset support

- Implement sub-asset import for mesh/model assets with manifest generation and deterministic GUIDs
- Extend AssetCatalog for sub-asset tracking and management
- Update AssetRegistry and ImportCoordinator for sub-asset workflows
- Add mesh asset parsing, GPU upload, and resource management
- Update mesh data structures for meshlet groups/hierarchy and LODs
- Improve tests for sub-asset import and mesh handling
- Enhance mocks for mesh asset testing and resource mapping
- Fix path handling and native DLL loading issues
- Miscellaneous bug fixes and refactoring

src/Runtime/Ghost.Generator/AssetHandlerRegistrationGenerator.cs

@@ -141,6 +141,11 @@ internal static partial class {registerTypeName}
             return null;
         }
 
+        if (symbol.IsAbstract)
+        {
+            return null;
+        }
+
         var iSettingsSymbol = context.SemanticModel.Compilation.GetTypeByMetadataName("Ghost.Editor.Core.Assets.IAssetSettings");
         if (iSettingsSymbol == null)
         {