Fix/shell saddle corners (#1316)

Thanks @Ian-cmd-ops.

tests/robustness/goldens.json

@@ -84,7 +84,7 @@
   "80": {"area":8652.533342,"bodies":1,"edges":20,"error":null,"faces":9,"outcome":"ok","rejected":0,"shells":1,"steps":["box","fillet","fillet"],"unsupported":0,"volume":37387.449469},
   "81": {"area":4001.847983,"bodies":2,"edges":75,"error":null,"faces":29,"outcome":"ok","rejected":1,"shells":2,"steps":["revolved section","cut sphere","circular pattern","rectangular pattern","hole","join cylinder","join box","cut sphere"],"unsupported":2,"volume":13139.658749},
   "82": {"area":3195.487112,"bodies":1,"edges":30,"error":null,"faces":13,"outcome":"ok","rejected":0,"shells":1,"steps":["box","hole","circular pattern","cut box"],"unsupported":0,"volume":5902.162954},
-  "83": {"area":5125.0,"bodies":1,"edges":28,"error":"offset: a corner whose faces don't meet the same way after t","faces":12,"outcome":"failed","rejected":0,"shells":1,"steps":["box","join box","rectangular pattern","shell"],"unsupported":0,"volume":19250.0},
+  "83": {"area":8489.059183,"bodies":1,"edges":97,"error":null,"faces":35,"outcome":"ok","rejected":0,"shells":2,"steps":["box","join box","rectangular pattern","shell","fillet","cut sphere","join box"],"unsupported":1,"volume":6217.394627},
   "84": {"area":4906.192237,"bodies":1,"edges":67,"error":null,"faces":25,"outcome":"ok","rejected":2,"shells":1,"steps":["extruded profile","fillet","hole","hole","cut cylinder","cut box","fillet"],"unsupported":0,"volume":14219.865875},
   "85": {"area":3309.210936,"bodies":1,"edges":57,"error":null,"faces":24,"outcome":"ok","rejected":1,"shells":2,"steps":["revolved section","fillet","cut sphere","circular pattern","join cylinder","hole","join box"],"unsupported":0,"volume":6904.455565},
   "86": {"area":7885.256491,"bodies":1,"edges":66,"error":"offset: a corner whose faces don't meet the same way after t","faces":28,"outcome":"failed","rejected":0,"shells":3,"steps":["extruded profile","join cylinder","join cylinder","circular pattern","rectangular pattern","shell"],"unsupported":2,"volume":21498.10695},
@@ -122,6 +122,7 @@
   {"reason":"#391 after merging main with #1213: seed 41 now reaches its second fillet (built since #1213), and the end-snap fix again drops a sliver edge the cuts left (94 to 93 edges); its steps, outcome, faces and shells are unchanged, volume moves by 0.005 mm³ (5185.215737 to 5185.221068) and area by 0.0014 mm²","seeds":[41]},
   {"reason":"the chamfer bug is on main too, see #1390","seeds":[57]},
   {"reason":"#1390: a chamfer of a convex edge that would grow the part is refused (TooLarge). Seed 57's 0.5 mm chamfer on the sphere cut's rim, which grew the part since #391 reordered its steps, is now refused, so the run goes on to its later steps and ends ok (16017.21 mm³, 24 faces)","seeds":[57]},
+  {"reason":"Seed 83 builds: a shell splits a saddle corner (convex and concave edges in turn) into two corners","seeds":[83]},
   {"reason":"seed 15 now cuts around a sealed void","seeds":[15]},
   {"reason":"#1490: when the last-resort nudge fails both ways, two more screw axes are tried; seed 63's final box join (its face 0.00005 mm off the half-ring's cut face) now builds instead of being refused with an implausible volume, and the design goes on to its fillet","seeds":[63]}
  ]