walkOneDir's check was hiding the worker's: a worker that walked a
queued directory on a cancelled scan still emitted nothing, because
walkOneDir stopped at its first entry. The worker test now queues a
missing directory, whose read fails and sends a warning before
walkOneDir's check is reached. A new test calls walkOneDir directly on
a cancelled scan and checks it returns no subdirectory to descend into.
Model: opus-5-5
sneak