The documented one-liner, iex (irm .../install.ps1), runs the installer
inside the user's own session. Fail ended with exit 1, so any failed
stage closed the user's PowerShell window.
Fail now throws. The two entry points own reporting and the exit code:
-Stage prints the reason, emits the -Json frame and exits 1, as before;
the full install exits 1 only when it runs from a script file, and under
iex it prints the reason, sets LASTEXITCODE=1 and returns. A scriptblock
literal's File tells the two apart: $MyInvocation.MyCommand.Path names the
caller's script under iex.
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