Fail ends the script with `exit 1`, which unwinds past the stage
dispatcher's try/catch, so the catch that frames failures as JSON never
ran for the installer's own fatal errors: a `-Stage repository -Json`
run whose clone failed printed the reason via Write-Host only and put
zero frames on stdout (verified on Windows: 0 frames before, 1 after).
Only thrown exceptions were framed.
Fail now emits the failure frame itself when running under -Stage -Json,
so every fatal path yields exactly one frame carrying the original
reason, matching install.sh's EXIT-trap framing.
3c53fbdc8b