Observe_fs_lwt.Make completes the portable filesystem writer with Lwt and an application-owned Observe_fs_lwt.IO.S. It adds promises, protected waiters, worker ownership, and cancellation behavior without adding Unix or lwt.unix.
This is the intended Mirage composition point. Supply filesystem capabilities from the target I/O implementation, apply the functor, and retain the returned writer so the application lifecycle can call flush and shutdown.