event_loop: don't debug-panic on late main-VM enqueue_task_concurrent during process.exit - #36020

The bounded poll loop at lines 68-72 falls through silently on deadline expiry — if `Date.now()` reaches `deadline` without `fs.readSync` returning >0, the fixture proceeds straight to `process.exit(0)` without asserting the writer ever entered the kernel. Per REVIEW.md's "assert that setup created the precondition" rule, track a `let sawBytes = false;` flag set inside the `> 0` branch and, after the loop, `if (!sawBytes) { process.stderr.write("writer never entered kernel\n"); process.exit(1);