Small isolated artifacts (e.g. a room corner the photo caught past the black
sheet) survived the flood-fill as separate slab islands and carried into the
rest of the workflow. After the border flood-fill, label the 4-connected slab
components and keep only the largest, with a 0.5%-of-frame area floor that
also drops speckles. Adds keepLargestComponent + unit tests proving an
isolated blob and a corner artifact are dropped while the main slab is kept.
Ty
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