The green/black-sheet luminance threshold could not separate the slab from
the 'black' sheet, which in real photos reads at lum ~50 and overlaps the
slab's bark/shadow tones (measured on the user's samples: 0% green, 60-80%
dark, no brightness gap) - it removed 100% of the image. Replace the mask
pass with connectivity-based background removal: sample the sheet tone from
the border, flood-fill from the border marking sheet-connected pixels as
background, keep the connected remainder as the slab. Robust to tonal
overlap. Adds bgRemoval.ts (removeBackground + sampleSheetTone) with unit
tests proving a sheet-toned slab pixel survives via enclosure.
Ty
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