tolerancePercent

Share of pixels this one fixture may differ by, or null — the normal case — to be judged by whatever the run itself is set to (viddik.tolerancePercent, otherwise DEFAULT_TOLERANCE_PERCENT).

Nullable rather than defaulted to the engine's own number so that "this fixture states its own budget" stays distinguishable from "this fixture says nothing": the engine's default belongs to the run, and a component carrying a copy of it would pin a fixture to today's value.