An RGBA_F32 image with an sRGB tag comes back off the GPU backend sampled on a
1/255 grid; the same shader on a raster surface returns the floats untouched.
The plane is raw/65535, so the shadows the black level is there to keep sit at
1e-3 and quantise to zero -- a 3010x2012 develop landed 41189 pixels under luma
2 with the dark end speckled blue/yellow, against none on the raster surface.
A half is uploaded as float, so the plane stays exact either way.
Rejects the earlier guess that the render target's colour space was to blame:
gpu+rt-srgb and gpu+img-untagged came back byte-identical to gpu.
scripts/half-check.mjs checks the conversion: the named encodings, and no plane
value in a 14-bit sensor's range moving more than 4.8e-4 relative.
3dtours
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