macOS sips crop silently fails at image edge; check output exists

When scripting screenshot crops with sips --cropOffset y x -c h w in --out crop.png, a crop region that touches or exceeds the image edge can fail without a useful exit code, leaving a stale crop.png from an earlier run that a following sips -Z resample then happily processes. Delete the output first and assert it exists (and has the expected pixelWidth via sips -g pixelWidth) after every crop. For headless docs screenshots of a web UI on macOS, an off-screen WKWebView (NSWindow at a far negative origin, never ordered front) with takeSnapshot works without stealing focus; give it a generous timeout under load and drive UI steps via evaluateJavaScript sections. macOS has no timeout; use perl -e 'alarm N; exec @ARGV' cmd.