installer-tests.yml predates nothing it still owned. Its pytest step
(test_source_launcher_stages.py) is platforms("windows") and already runs in
both tests-os Windows lanes, so every installer PR ran it twice. The
`installer` lane never gated anything on its own either: every path that set
it also sets `python`, which gates tests-os.
The two standalone scripts/tests/*.ps1 suites become one platforms("windows")
pytest file parametrized over Windows PowerShell 5.1 and pwsh 7, so
list_os_marked_tests picks them up with everything else. The `installer` lane
goes away from the classifier, detect-changes, ci.yaml and the
all-checks-pass gate; the classifier contract now pins that install.ps1 and
its suites turn `python` on.
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