Feedback lands before the tab switch

Homework feedback usually runs on the grader's clock: submit Friday, learn Monday what you got wrong, having forgotten why you wrote it that way.

Each student's container carries a file system watcher on the exercise folder. The moment a student saves, the watcher emits the change event; a small subscriber POSTs the file to an exec script that runs the exercise's tests; the result comes back as a notification in the student's own environment. Pass or fail, with the failing assertion named, seconds after the keystroke.

The student is still looking at the line. That's the moment the correction teaches something. Forty containers means forty private feedback loops, none of them sharing a queue.

The red X arrives while the mistake is still on screen.