Address @vanceingalls review on #848: the mp4-h264-sdr sibling fixture
explains the crossfade-straddles-frame-30 and continuous-rotation
chunk-seam design choices inline; mov-prores didn't. Add the parallel
comment so the next contributor reading either fixture finds the same
context. Notes specifically that ProRes is intra-only and therefore
exercises the QuickTime atom / -c copy contract rather than frame-level
state continuity.
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