Failing furnace test

[pmudry]

Any deviation from pure white on any of these spheres pinpoints where energy is being lost (dark artifacts) or gained (bright artifacts). That's the diagnostic power of the furnace test.

The shadow on the right side of the top-left sphere likely points to an issue with how the cosine-weighted hemisphere sampling or the MIS balance heuristic handles one-sided illumination bias — worth investigating in the material dispatch or PDF computation code.