seams at union edges

[greysAcademicCode]

seams between fused parts remain in the fuse output when the items being fused share some common edge/face

note to self: look into how tolerance impacts this and look at "fuzzy" fuse:
https://github.com/FreeCAD/FreeCAD/blob/898e0fb359c143bea4ce93d0ef85d0c839ed3dce/src/Mod/Part/App/TopoShape.cpp#L1373