After splitting a drawing with a circular hole, CadConverterForm writes
the split piece to DXF and re-imports it. The circle (decomposed into
two semicircular arcs by DrawingSplitter) was being incorrectly merged
back into a single zero-sweep arc by GeometryOptimizer.TryJoinArcs
during reimport.
Root cause: TryJoinArcs mutated input arc angles in-place and didn't
guard against merging two arcs that together form a full circle. When
arc2 had startAngle=π, endAngle=0 (DXF wrap-around from 360°→0°), the
mutation produced startAngle=-π, and the merge created an arc with
startAngle=π, endAngle=π (zero sweep), losing half the hole.
Fix: use local variables instead of mutating inputs, require arcs to be
adjacent (endpoints touching) rather than just overlapping, and refuse
to merge when the combined sweep would be a full circle.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
