Source-level guard (install.ps1 only runs on Windows, so there's no Linux CI
runner to execute it): the astral uv install line must be invoked via the call
operator on a resolved host variable, the bare-`powershell` literal that
produced the field-reported "The term 'powershell' is not recognized" must be
gone, and the resolver must be PATH-independent (Get-Process -Id $PID) and
pwsh-aware.
