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/Editor/Ghost.Editor/ActivationHandler.cs

@@ -61,7 +61,6 @@ internal static class ActivationHandler
             StackCapacity = 1024 * 1024 * 32, // 32 MB. Stack using virtual memory, so this is just a reservation and won't actually consume physical memory until used.
             FreeListChunkSize = 64 * 1024 * 1024,
             FreeListDefaultAlignment = 8,
-            FreeListConcurrencyLevel = Environment.ProcessorCount
         };
 
         AllocationManager.Initialize(opts);