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/Runtime/Ghost.Core/Ghost.Core.csproj

@@ -8,7 +8,7 @@
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
-    <DefineConstants>$(DefineConstants);MHP_ENABLE_SAFETY_CHECKS;MHP_ENABLE_MIMALLOC;MHP_FASTMATH;MHP_ENABLE_STACKTRACE</DefineConstants>
+    <DefineConstants>$(DefineConstants);MHP_ENABLE_SAFETY_CHECKS;MHP_ENABLE_MIMALLOC;MHP_FASTMATH</DefineConstants>
     <IsAotCompatible>True</IsAotCompatible>
     <IsTrimmable>True</IsTrimmable>
   </PropertyGroup>

src/Runtime/Ghost.Core/Properties/launchSettings.json

@@ -0,0 +1,8 @@
+{
+  "profiles": {
+    "Ghost.Core": {
+      "commandName": "Project",
+      "debugEngines": "managed"
+    }
+  }
+}

src/Runtime/Ghost.Engine/AssetLoader/Asset.cs

@@ -19,6 +19,16 @@ public readonly struct AssetReference : IEquatable<AssetReference>
     public readonly bool IsInternal => _value >= 0;
     public readonly bool IsExternal => _value < 0;
 
+    public AssetReference(int index, bool isInternal)
+    {
+        if (index < 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(index), "Index must be non-negative");
+        }
+
+        _value = isInternal ? index + 1 : -(index + 1);
+    }
+
     public bool Equals(AssetReference other)
     {
         return _value == other._value;