ffmpeg prints "corrupt decoded frame" as a warning, so the oracle's
`-v error -ec 0` decode let a stream with a whole slice missing pass
silently. Switching that decode to `-v warning` is not enough: with
`-ec 0`, ffmpeg 7.1/8.1 flags every picture that has more than one
slice, libx264's multi-slice streams included, while the pixels are
unchanged. That is the 119/120 in #74; the encoder's bitstream is not
at fault (VideoToolbox decodes the same streams bit-identically, and
ffmpeg with strict error detection reports nothing).
The oracle now decodes with ffmpeg's default concealment, `-v warning`
and `-err_detect +crccheck+bitstream+buffer+explode`, fails on any
output or a non-zero exit, and still compares every pixel with the
encoder's reconstruction. A negative control drops and truncates
slices (CAVLC and CABAC, 3 slices) and checks the oracle reports each
one; it fails with the old flags.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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