Function drain_queue

Apply every queued mutation to disk, in FIFO order, until the queue is empty, then mark the worker idle. Spawned once via [`kick_worker`] and runs as the sole mutator; the awaited read paths wait on a [`Mutation::Barrier`] rather than draining themselves, so there is never more than one drainer. Consecutive appends to the same path are coalesced into one combined append. Each `createWritable` on

(inner: Arc<Mutex<Inner>>)