Root cause: resources and default mount stored pointers tied to the public mjVFS object address. If mjVFS was moved after initialization, resource close/read paths could dereference stale pointers and crash.\n\nThis change stores a stable internal mjVFS handle inside VFS and points resources/default mount to that stable handle. It also tracks the current public owner pointer and rebinds it in Upcast so lifecycle operations (including self-destruct cleanup) target the current mjVFS address after moves.\n\nAdds regression test UserVfsTest.MoveVfsAfterOpenResource, which opens a resource, moves mjVFS, then reads/closes/deletes successfully.
Nandini Dhanrale
c45f13d3a9