Encode quality control gate

Design the automated encode quality-control (QC) gate that sits between the transcoding farm and publishing, catching bad encodes before viewers see them across millions of encodes/day. Failures to catch include: silent corruption (green frames, macroblocking), audio/video drift, missing or truncated renditions, wrong aspect ratio/letterboxing, loudness out of spec, and a rendition that's perceptually worse than a lower rung. The gate must be automated (humans can't watch every encode) but escalate genuinely ambiguous cases.