FileWatching: fix data race on timedout flag in poll_fd

The `timedout` flag in `poll_fd` was a plain `Ref{Bool}` accessed from
both the Timer callback and the calling task without synchronization.
Replace it with `Threads.Atomic{Bool}` to eliminate the data race.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>