- An edge's side is taken only when a step either way tells inside from
  outside. A wall thinner than the step is "unknown" (2 in native), and no
  seam is skipped at its edges, so duplicated thin walls keep their corners.
- Floors are cleared and refilled with antialiasing, and the floor layer is
  added rather than laid over: at a floor edge on lit ground the cut keeps
  1 - c and the floor brings c, which sum to the fill. Edges facing open
  ground stay smooth.

A ray grazing a corner from within about 1e-5 SVG units of it is still
caught by the slack's 1e-13 floor; a smaller floor would let roundoff send
a corner-aimed ray through the wall, which is worse.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>