Last commit added `_color_match` which shifts the face crop's mean to the
canvas mean -- the old test fed a uniform face (210) into a uniform cleaned
canvas (90), so after color-match the face was uniform 90 and the
composite was undetectable by value. Switched the fake pipeline to a
gradient face so the color-match preserves variance, and the assertion
now checks that the face region has non-zero std (composite injected
gradient pixels) instead of a value threshold.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>