Function lower_masks 

pub fn lower_masks(
    data: &EdgeFeatureData,
    child: &DynamicObject,
) -> (LoweredMasks, LowerReport)

The masks for data against child, in the child’s frame. Unresolvable/unsupported edges are skipped and counted.

Edges resolve against the unfeatured child (inner chamfers/fillets stripped): a sibling feature’s cut must not trim a stored edge’s surviving run, or the mask stops short of shared corners and a sliver of the original sharp edge pokes through where mixed-kind features meet. Full-length masks simply overlap at corners, matching the same-kind merge behavior.