`fn snapshot` wrote every reply but never looked at `ok`, so a script with a
bad `ui.set`, an unknown command or a failed `ui.screenshot` exited 0 while
the PNG showed the wrong view or was never written. The run now counts
replies with `"ok": false`, keeps going (every reply still reaches stdout and
the final `-o` screenshot is still written — continue-and-report, like
`run --keep-going`) and returns `N scripted request(s) failed` afterwards,
which `main` turns into exit 1. Usage text and docs/control-protocol.md
"Headless rendering" state the contract.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>