Scoping a supervised dashboard/serve backend's background processes moved them out of the
backend's service cgroup, so they survived a backend restart, but nothing in dashboard/serve
startup recovers the process checkpoint: they kept running while process list/poll/kill could
not find them, and the next spawn dropped their checkpoint record. Recovering them in serve
would conflict with the gateway owning background processes (#106742). Revert to main's
behaviour for dashboard/serve (workers stay in the backend's cgroup and die with it) and leave
escapee cleanup there to the serve-to-gateway cutover.
Tests: the escapee in the hang test printed before the kill deadline, which ended the reader's
blocked read and let the old close finish in time, so the test passed without the fix. The hang
test's escapee is now silent; late-output rejection has its own test. Removing the reader-close
guard now fails the hang test.
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