Introduce explicit VFS in Python MjVfs

This change introduces MjVFS, an explicit object to mirror the C mjVFS. This is meant to replace the MjSpec.assets dict. This latter while convenient guides users towards harmful authoring patterns with respect to data duplication and spec attachment workflows. Making VFS management explicit should encourage better memory usage and allow us to make better compile time optimizations.

From this change, `spec.assets` is deprecated. However we will temporarily support backwards compatibility due to the wide spread usage. An error will be thrown if calling code tries to use a spec that uses both the assets dict and the new MjVfs.

PiperOrigin-RevId: 908772050
Change-Id: I77c6d0369307fc300c954768fed17401261e18da

python/mujoco/vfs.h

@@ -0,0 +1,46 @@
+// Copyright 2026 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef MUJOCO_PYTHON_VFS_H_
+#define MUJOCO_PYTHON_VFS_H_
+
+#include <memory>
+
+#include <mujoco/mujoco.h>
+
+namespace mujoco::python {
+
+class MjVfs {
+ public:
+  MjVfs() : vfs_(new mjVFS) { mj_defaultVFS(vfs_.get()); }
+
+  void Close() { vfs_.reset(); }
+
+  mjVFS* get() const { return vfs_.get(); }
+
+  bool is_open() const { return vfs_ != nullptr; }
+
+ private:
+  struct VfsDeleter {
+    void operator()(mjVFS* vfs) const {
+      mj_deleteVFS(vfs);
+      delete vfs;
+    }
+  };
+  std::unique_ptr<mjVFS, VfsDeleter> vfs_;
+};
+
+}  // namespace mujoco::python
+
+#endif  // MUJOCO_PYTHON_VFS_H_