Probes the rendered output for video and audio stream durations after
render and fails the test if they differ by more than 0.5s. Catches
mux-level truncation regressions like the ffmpeg -shortest bug (#1648)
where one stream gets silently cut short.
Runs on all non-png-sequence fixtures with audio — no new meta.json
field needed since this is a universal invariant, not a per-fixture
threshold.
