Add ArcFit.FitWithDualTangent to constrain replacement arcs to match
tangent directions at both endpoints, preventing kinks without
introducing gaps. Add DXF year selection to CAD converter export.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Move identical FitWithStartTangent and MaxRadialDeviation methods
to a shared ArcFit class, eliminating 40-line duplicate.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
