EllipseConverter, SplineConverter and GeometrySimplifier each carried an
identical private SumSignedAngles. Move the unchanged body to
ArcFit.SumSignedAngles and call it from all three, keeping the ordered
accumulation, strict half-turn comparisons and empty/single-point result.
ArcFitTests compares the shared method bit-for-bit with a separate
test-local accumulator across half turns, the atan2 seam, multiple
turns, translated centres and NaN inputs, and checks that inputs are not
mutated. The converter winding characterization from 8664656 still
passes unchanged.
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