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Atlas dark-feature sampling baseline
This folder preserves the pre-change src/shaders/mosaic.frag as baseline-mosaic.frag so the Option2 atlas behavior remains reproducible.
The focused change probes four fixed cardinal offsets inside one 8x8 atlas texel footprint during REGION_ATLAS construction. Each probe uses the existing projected scene UV for both source RGB and front-depth evidence. A probe is accepted only when its .80-.95 front-depth and .10-.20 luminance evidence are stronger than the center sample; the representative blend is capped at .4. Center, far, invalid, and uniform cells retain the old result.
The added regression is a source/math-only check in scripts/test-region-atlas-dark-sampling.mjs; it does not claim browser visual success. The extra cost is four RGB probes per atlas texel, plus the existing validity/evidence lookups for those probe UVs. The final display pass, palette fitting, optical path, history, and camera gain are unchanged.