A boolean that cuts a pocket into the top of a cylinder (Shell does this) splits the
outer wall's straight seam edges at the pocket's floor. The wall's parameter grid has no
division along its rulings, so the triangulation fanned out from the two split vertices
to the far rims and its triangles cut through the part: the shelled Bolted Flange sample
drew and exported its rim cut off (mesh volume 29201 mm3 for a 40796 mm3 body).
Give the grid a line through each vertex that splits a boundary segment running along a
ruling, in a direction that needs no division of its own and within the grid budget.
Fixes#819.
Co-authored-by: Claude Opus 5.5 <[email protected]>
