Pure client modules for TECH-SPEC sheet detect, chroma-key / black
threshold with border flood-fill, four knobs (live re-run), and mask
confirm hooks. Synthetic fixtures. U2Net never invoked.
TDD: red evidence in test-results/tv1-tv2-red.txt, then minimum impl.
Results:
TV-1 | pass | test-results/tv1-tv2-green.txt
TV-2 | pass | test-results/tv1-tv2-green.txt