Boolean: cut around a part's sealed voids when the whole cut fails (#1570)

A cut clear of every void cuts the outer shell alone and puts the voids back (robustness seed 15 now builds).

tests/robustness/goldens.json

@@ -16,7 +16,7 @@
   "12": {"area":4668.664174,"bodies":2,"edges":59,"error":null,"faces":25,"outcome":"ok","rejected":0,"shells":2,"steps":["extruded profile","chamfer","join cylinder","join box","cut cylinder","circular pattern"],"unsupported":3,"volume":11874.471611},
   "13": {"area":1967.991667,"bodies":3,"edges":85,"error":null,"faces":37,"outcome":"ok","rejected":1,"shells":3,"steps":["revolved section","cut cylinder","join box","rectangular pattern","cut cylinder","cut box","fillet","hole"],"unsupported":0,"volume":2647.948638},
   "14": {"area":10553.267409,"bodies":1,"edges":43,"error":null,"faces":20,"outcome":"ok","rejected":0,"shells":1,"steps":["extruded profile","cut cylinder","shell","fillet"],"unsupported":0,"volume":10369.008764},
-  "15": {"area":1974.784048,"bodies":1,"edges":40,"error":"boolean Cut: the operation failed: no result; the operation ","faces":18,"outcome":"failed","rejected":1,"shells":2,"steps":["revolved section","fillet","cut box","rectangular pattern","cut sphere","cut cylinder","cut sphere"],"unsupported":0,"volume":4781.224576},
+  "15": {"area":1957.859861,"bodies":1,"edges":49,"error":null,"faces":21,"outcome":"ok","rejected":1,"shells":2,"steps":["revolved section","fillet","cut box","rectangular pattern","cut sphere","cut cylinder","cut sphere"],"unsupported":0,"volume":4723.415743},
   "16": {"area":8872.873884,"bodies":3,"edges":49,"error":null,"faces":23,"outcome":"ok","rejected":0,"shells":3,"steps":["box","join cylinder","circular pattern","hole"],"unsupported":0,"volume":44953.403148},
   "17": {"area":6048.425782,"bodies":1,"edges":48,"error":null,"faces":19,"outcome":"ok","rejected":0,"shells":1,"steps":["revolved section","cut box","shell","cut cylinder"],"unsupported":0,"volume":2968.53154},
   "18": {"area":5524.583817,"bodies":4,"edges":98,"error":"boolean Cut: the operation failed: no result","faces":44,"outcome":"failed","rejected":0,"shells":4,"steps":["extruded profile","join cylinder","circular pattern","cut cylinder","hole","cut cylinder","hole","circular pattern"],"unsupported":0,"volume":21568.016672},
@@ -121,6 +121,7 @@
   {"reason":"#1213: a round or chamfer on an edge chain that runs on across a curved seam now builds (it was refused with 'a curved seam where the edge chain turns'). Seed 41's second fillet (three edges, radius 1) was one of those refusals and is now built. Replaying the seed's commands on the build before and after gives the same volume, area, faces and edges after each of steps 1 to 6; step 7, the fillet, is refused before and builds after (5292.19 to 5289.17 mm³, area 7395.26 to 7381.22 mm²); the run then reaches its eight steps one step sooner, so its last step is a cut sphere (5185.22 mm³) where it was a join cylinder (the generator draws its later steps from the new state).","seeds":[41]},
   {"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":"#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 15 now cuts around a sealed void","seeds":[15]}
  ]
 }