Scale arc tolerance for JoinTypeRound in Clipper offset

The Clipper default arc tolerance is 0.25 lattice units, which is `0.25 fm` for float inputs. This creates ~10^5 vertices for offsets with JoinTypeRound at micron scale. We should set `ArcTolerance`, either to scale with `SCALE`, our default `1 nm` tolerance, or the Clipper2 default ArcTolerance `offset_radius / 500`. This would be a bugfix.

Start at the Clipper offset path for JoinTypeRound and trace how ArcTolerance, SCALE, and float inputs interact. Compare the proposed tolerance rules at micron scale, then verify that the chosen behavior avoids excessive vertices while preserving the intended round offset geometry.

Written by the indexing model from the issue text.