The spec review found two places where the marker and threshold
contract does not hold.
A clamp narrower than the chord tolerance is still judged
tolerance-free against finely re-read material and reported as the
whole clamp rectangle. That existing safety policy is kept, so its
marker is the clamp's centre: within half the clamp's size (under
0.0005) of the material, though not necessarily on it. Document this
on PlateOverlapPair.Marker and in the overlap docs, and lock it in the
tiny-clamp region test.
Also document that 1e-8 grid rounding can move an area within
rounding error of the 1e-5 threshold to either side of it.
aj
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