Read clamps.json as a raw document so an explicit null is reported as
an ignored value (Partial, which the desktop warns about) instead of
being taken as an absent field. Missing or invalid sizes now fall back
to the physical 1.5 x 0.625 inch clamp expressed in the file's units,
so a millimeter file no longer gets a 1.5 x 0.625 mm clamp. Values that
would overflow when converted between inches and millimeters are
ignored and reported instead of throwing while creating a nest.
Review findings 2, 3 and 6, Project Memory 4e5f566d.