Refactor mesh asset handling and memory allocation

- Unified FBX/OBJ logic into MeshAssetHandler and moved mesh node classes to MeshNode.cs
- Updated IAssetHandler to use CreateDefaultSettings(string ext)
- Made MeshAsset the abstract base, removed FBXAsset
- Switched mesh import/processing to use memory pools and explicit AllocationHandle
- Standardized manifest serialization options
- Improved error handling and normalized project paths
- Updated tests, project files, and AssetReference struct

src/Test/Ghost.UnitTest/AssetSystem/MeshAssetHandlerTests.cs

@@ -65,9 +65,9 @@ public class MeshAssetHandlerTests
 
         var parentGuid = Guid.NewGuid();
         var targetPath = ImportCoordinator.GetImportedAssetPath(parentGuid);
-        var handler = new FBXAssetHandler();
+        var handler = new MeshAssetHandler();
 
-        var result = await handler.ImportWithSubAssetsAsync(sourcePath, targetPath, parentGuid, new ObjAssetSettings(), TestContext.CancellationToken);
+        var result = await handler.ImportAsync(sourcePath, targetPath, parentGuid, new ObjAssetSettings(), TestContext.CancellationToken);
 
         if (result.IsFailure && result.Message?.Contains("Unable to load DLL", StringComparison.OrdinalIgnoreCase) == true)
         {