Fillet and chamfer: refuse a smooth seam as having no corner to round (#936)

* Fillet and chamfer: refuse a smooth seam as having no corner to round

Picking the seams where R10 rounds run into the faces beside them, and asking for an R1
fillet there, failed with "not supported yet: blending a smooth edge (... both faces next
to the edge must be planar)". That reads like a missing feature, but the faces meet
tangentially at those edges and there is nothing to round.

An edge whose two faces are tangent along it is now refused as an invalid pick that names
the edge and asks for the sharp edges.

Fixes #896.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

* Robustness goldens: seed 72's smooth-seam fillet is now a rejected pick, not unsupported

Co-Authored-By: Claude Opus 5.5 <[email protected]>

---------

Co-authored-by: Claude Opus 5.5 <[email protected]>

tests/robustness/goldens.json

@@ -73,7 +73,7 @@
   "69": {"area":6451.122768,"bodies":1,"edges":132,"error":"offset: a corner whose faces don't meet the same way after t","faces":27,"outcome":"failed","rejected":0,"shells":3,"steps":["revolved section","join box","shell","shell"],"unsupported":0,"volume":4761.477514},
   "70": {"area":3682.669157,"bodies":1,"edges":24,"error":"lumps: shell boundaries intersect or touch; cavity ownership","faces":12,"outcome":"failed","rejected":0,"shells":2,"steps":["extruded profile","join box","hole"],"unsupported":1,"volume":9155.027162},
   "71": {"area":3980.38113,"bodies":1,"edges":50,"error":null,"faces":19,"outcome":"ok","rejected":0,"shells":1,"steps":["revolved section","cut box","cut cylinder","hole","join cylinder"],"unsupported":0,"volume":15750.655245},
-  "72": {"area":5770.270336,"bodies":2,"edges":46,"error":null,"faces":20,"outcome":"ok","rejected":0,"shells":2,"steps":["box","join cylinder","hole","join box","join box","cut box","cut cylinder","cut cylinder"],"unsupported":1,"volume":16081.801492},
+  "72": {"area":5770.270336,"bodies":2,"edges":46,"error":null,"faces":20,"outcome":"ok","rejected":1,"shells":2,"steps":["box","join cylinder","hole","join box","join box","cut box","cut cylinder","cut cylinder"],"unsupported":0,"volume":16081.801492},
   "73": {"area":6868.162117,"bodies":1,"edges":40,"error":"offset: a corner whose faces don't meet the same way after t","faces":18,"outcome":"failed","rejected":0,"shells":1,"steps":["extruded profile","join box","shell"],"unsupported":1,"volume":36461.939381},
   "74": {"area":9294.57311,"bodies":1,"edges":22,"error":null,"faces":9,"outcome":"ok","rejected":0,"shells":1,"steps":["box","hole"],"unsupported":0,"volume":55866.183074},
   "75": {"area":7893.117931,"bodies":1,"edges":27,"error":null,"faces":11,"outcome":"ok","rejected":0,"shells":1,"steps":["revolved section","cut cylinder","chamfer","rectangular pattern","rectangular pattern","cut box"],"unsupported":0,"volume":38757.62083},
@@ -101,6 +101,7 @@
  "notes": [
   {"reason":"First record: what each seed builds on main at 1a0100cd","seeds":[0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95]},
   {"reason":"seed 0 meshes differently since display tessellation plans (#805), which landed alongside these goldens (#826)","seeds":[0]},
-  {"reason":"the kernel fixes merged together on 13 October (#838, #839, #841, #855, #856) change what these seeds build: several joins that used to fail now succeed, small volume and area shifts, and shell now refuses seeds 6, 14, 43 and 73 with an offset corner error (seed 14 built before: #897)","seeds":[2,6,14,22,36,43,50,63,71,73,94]}
+  {"reason":"the kernel fixes merged together on 13 October (#838, #839, #841, #855, #856) change what these seeds build: several joins that used to fail now succeed, small volume and area shifts, and shell now refuses seeds 6, 14, 43 and 73 with an offset corner error (seed 14 built before: #897)","seeds":[2,6,14,22,36,43,50,63,71,73,94]},
+  {"reason":"#936: a fillet or chamfer of a smooth seam is refused as an invalid pick (no corner to round) instead of as unsupported","seeds":[72]}
  ]
 }