Moved in by more than its radius, a round collapses and its neighbours meet
along a new edge. That edge was always built as a straight line keyed by its
ends, so on a disc whose circular edge is rounded the four arcs where the floor
meets the side became one chord used by four faces, and the shell failed with
"This shell is not oriented and closed" (#319).
The new edge now runs where the edge's face meets the face across the round
(the round's far side, crossed as a strip), through the edge's middle moved
onto both: a line or a circular arc, checked to lie on both moved faces, and
refused otherwise. Twins are the new edges with the same ends through the same
middle.
Fixes#319
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
