fix: shell bodies where four or more planar faces meet at a corner (a floor falling to a drain from the trough edges) by splitting such corners into several joined by short edges instead of refusing because the moved planes miss a common point

ROADMAP.md

@@ -100,7 +100,8 @@ headline counts in-scope tabs only and lists the deferred ones separately.
 - Free-form curves cut at a tangent touch (no clean crossing) enter that profile as polylines.
   Sweeps along 3D sketch curves follow their polyline (smooth walls when the path is smooth).
 - Shell and offset faces work on bodies of planes, cylinders, cones and spheres (convex or not:
-  filleted boxes, bosses, domes, countersinks); draft on convex planar bodies and on walls (planes, cylinders → cones) between caps; lofts through three or more sections are smooth (B-spline sides through every section), two
+  filleted boxes, bosses, domes, countersinks; corners where four or more planes meet, such as a
+  floor falling to a drain, as long as the corner's edges are all convex or all concave); draft on convex planar bodies and on walls (planes, cylinders → cones) between caps; lofts through three or more sections are smooth (B-spline sides through every section), two
   sections ruled.
 
 ## Kernel