- A shared edge cancels at a vertex only when the two walls lie on either
  side of it. Two walls over the same footprint, wound opposite ways, no
  longer lose their corners. Dart works out each edge's inside from its
  wall and hands it to native (ish_set_interior_sides); without it native
  skips no seams.
- Corner slack is 1e-13 + 1e-10 x the hit distance along the wall, so a
  ray 1e-8 radians beside a corner still passes it from a millimetre away.
- Floors are cleared and refilled without antialiasing, by the same pixel
  centres, so no faint seam is left along a floor edge inside a cone.

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