On a real update the panel stayed on 'Installing the new app' after the
app relaunched: the shim publishes 'done' and then removes the status
file, and the panel treated a missing file as 'keep waiting' — it only
ever exited if a poll happened to land in the half-second window before
the removal. Race lost, panel immortal.
A disappearance AFTER the file has been seen to exist now means done
(the shim removes the file only after publishing a terminal state and
tearing down its UI; an error publish keeps the file alive through the
15s leave-window grace, so a failure cannot be masked). A file that
never existed still means 'no status yet'. stop_ui also resets
UI_PANEL_PID with the other UI handles.
Verified on a macOS host: the panel compiles, survives while the file
exists, and self-exits ~3s after the file vanishes.
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