scripts/tests/ has held three PowerShell suites that no workflow ever
invoked -- there is no Windows runner in CI, so they have been inert since
they landed. A regression test nothing executes is worse than none: it
reads as coverage.
Adds a windows-latest job, gated on a new `installer` lane so it only fires
for PRs touching install.ps1 or its tests. The 8.3 suite runs under both
pwsh 7 and Windows PowerShell 5.1, since install.ps1 arrives via `irm | iex`
into whichever shell the user already has and 5.1 is what ships with Windows.
Only the 8.3 suite is wired up. The other two fail on main today for
unrelated reasons; they can join once they are fixed.
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