SkyViewLUT instances (backend:'wgsl' / 'tsl') sharing one Transmittance
+ Multi-Scatter LUT, each rendered to a float target and diffed. This exercises the whole integrator in WGSL —
medium sample, sun-transmittance (manual bilinear), earth shadow, MS-LUT feedback, power-series in-scatter, ground
bounce. Values are tiny (per-unit-sun radiance, ~0.001–0.04), so relative diff is reported too. The canvas shows
the WGSL SkyView LUT (×40).