benchImage and BenchmarkEncodeAtInputSize existed in both encoder packages, and
blurhash's copy had drifted into a third inline duplicate once both benches
moved to NRGBA input.
The head-to-head now lives in core/artwork, which already imports both encoders
and can pin the input to the real thumbnailSize constant. The gradient builder
moves to tests, which both packages already import via their suite bootstrap.
thumbhash keeps a shipped-vs-reference benchmark, since referenceEncode is
test-only and cannot be reached from core/artwork.
9084685fbc