The self-test grew a branch that spawns a Python child so pytest could prove
progress advances during one. It doesn't need to: /progress is answered from
its own runspace, so the existing hold already blocks the main thread, and
the spawn only exercised Invoke-HermesStep, which nothing here changes.
The Windows test now asserts the invariant instead of the self-test's stage
string, and the posix half -- previously untested, and the half that broke --
gets real coverage: serve-ui.py's wire shape, and posix.sh driven end to end
with a stub `hermes` that reports which stage was on screen while it ran.
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