Vec2 moves from math to geom::vector2d (re-exported from math). Back-edges
sm_sqrt/sm_acos/sm_asin and SM_IS_ZERO_SQUARED are co-ported as private
helpers. difftest: 4060 cases bit-identical.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>