The hand-off scripts gave the Desktop a fixed 30 s (windows.ps1) or 60 s
(posix.sh) to exit, but the Desktop's quit can first join a managed SSH update
it is running, which legitimately takes longer. The hand-off then recorded a
failed update although the Desktop exited safely moments later (review H2).
Both twins now share one 150 s ceiling, measured on the clock rather than as a
loop count, and still refuse with "Nothing was changed" if the Desktop is alive
at the ceiling, so nothing is ever replaced under a live Desktop. The ceiling is
overridable through HERMES_UPDATE_DESKTOP_EXIT_SECONDS only so the self-tests
need not sit it out, like the existing pipe-drain and step-idle overrides.
Tests: a Desktop still alive at the ceiling refuses with exit 4 and never runs
the update (red on the start sha, which waited out its hardcoded 60 s); a
Desktop that exits inside the ceiling lets the update run (control).
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