Extract the pure world-space position/tangent math from the WinForms
cut-direction arrow renderer into Core (ContourSampler), keeping screen
conversion, arrowheads, and the per-move display policy in the view.
Add a contour-wide arclength scheduler (RingMoves) for measurement use:
distance carries across segment boundaries, short segments are never
omitted, and the closing vertex is not duplicated. Characterization
tests pin the arrow policy (counts, short-move skipping, CW/CCW and
full-circle sweeps, subprogram offsets, suppressed/rapid moves,
incremental mode) and the scheduler's start-vertex invariance.
