Every item left its SSIM windows (291 MB each), per-offset pairs, VMAF pairs and stats logs behind;
anything killed mid-item left them for ever. Measured 2026-10-12: 6 windows + 123 gate logs, work
drive at 3.9 GB, both lanes idle on the guard. process() now purges its own tag/pid artefacts in the
finally, and sweep_stage() clears leftovers older than STAGE_MAX_AGE_H while skipping any tag that has
a fresh <tag>_out.mp4.err (i.e. is encoding right now). Regression: scripts/test_stage_cleanup.py.