Detects pairs of consecutive 135° vertices (characteristic of filleted
90° corners) and reconstructs circular arcs from tangent-perpendicular
intersection. Verified on sandbox 2: 2 arcs detected at R=7.5mm with
correct centers. Chain continuity validated.
When arcs are detected, v1 boundaries get promoted to v2 typed segments
and the polyline is re-densified with proper arc interpolation.
