The three tiers run side by side by design (copy is I/O, x265 is CPU, nvenc is the GPU ASIC) and
they share the stage directory - but the gate's raw frame files had FIXED names, so two concurrent
gates overwrote each other's decodes. Measured: a comparison reported 100% of frames below 0.85 -
which reads exactly like a systematic defect and was nothing but two runs writing ssim_ref.yuv at
once. The raws are now tagged with the pid and the file tag. A single-file conversion failed its
gate on this and the 'defect' was entirely my own harness.