Blends: a curved seam where the edge chain runs on holds the contact on itself (#1291)

A round or chamfer whose edge chain runs on across a curved seam (the end circle of a lofted tube whose axis bends) is now built: the section solve keeps its contact on the seam through a plane taken afresh at the seam's nearest point at each step. Straight seams are unchanged. Robustness seed 41 is re-recorded: its second fillet, refused before, now builds. Fixes #1213.

Co-authored-by: gvocale <[email protected]>

tests/robustness/goldens.json

@@ -42,7 +42,7 @@
   "38": {"area":11769.375811,"bodies":1,"edges":30,"error":"offset: a corner whose faces don't meet the same way after t","faces":12,"outcome":"failed","rejected":1,"shells":1,"steps":["box","hole","cut cylinder","shell"],"unsupported":0,"volume":78261.020975},
   "39": {"area":1892.050815,"bodies":1,"edges":27,"error":null,"faces":11,"outcome":"ok","rejected":0,"shells":1,"steps":["extruded profile","chamfer","cut sphere","rectangular pattern"],"unsupported":1,"volume":4040.62787},
   "40": {"area":15548.516294,"bodies":1,"edges":49,"error":"offset: a corner whose faces don't meet the same way after t","faces":22,"outcome":"failed","rejected":0,"shells":1,"steps":["extruded profile","cut cylinder","shell","cut cylinder","join cylinder","shell"],"unsupported":0,"volume":16543.777532},
-  "41": {"area":7611.393446,"bodies":1,"edges":99,"error":null,"faces":35,"outcome":"ok","rejected":1,"shells":1,"steps":["box","cut sphere","shell","fillet","cut cylinder","rectangular pattern","cut sphere","join cylinder"],"unsupported":1,"volume":5973.93328},
+  "41": {"area":7295.281456,"bodies":1,"edges":94,"error":null,"faces":32,"outcome":"ok","rejected":1,"shells":1,"steps":["box","cut sphere","shell","fillet","cut cylinder","rectangular pattern","fillet","cut sphere"],"unsupported":0,"volume":5185.215737},
   "42": {"area":5486.877622,"bodies":1,"edges":51,"error":null,"faces":20,"outcome":"ok","rejected":0,"shells":1,"steps":["extruded profile","cut box","cut cylinder","rectangular pattern"],"unsupported":0,"volume":19095.106157},
   "43": {"area":3492.988343,"bodies":1,"edges":72,"error":null,"faces":26,"outcome":"ok","rejected":2,"shells":1,"steps":["extruded profile","cut cylinder","rectangular pattern","fillet","hole","rectangular pattern","join cylinder"],"unsupported":1,"volume":7528.548742},
   "44": {"area":7972.435566,"bodies":1,"edges":36,"error":null,"faces":15,"outcome":"ok","rejected":0,"shells":1,"steps":["extruded profile","shell","cut sphere"],"unsupported":0,"volume":7622.184366},
@@ -115,6 +115,7 @@
   {"reason":"corner blends on curved faces (#854): seed 33's last fillet (its second body) is built differently; its display-plan and measured volumes now agree (4442.73 mm³; on main they read 4439.42 and 4443.51)","seeds":[33]},
   {"reason":"#952: a hole placed on a face drills into the material; seed 81's hole moves its volume by 0.1 mm³ (area by 0.04 mm²)","seeds":[81]},
   {"reason":"Cuts join the straight edges they used to leave split along untouched walls (#1049). Replaying each changed seed's new commands on the build before and after gives the same volume, area and faces at every step, and the same refusals (seeds 2 and 95 end on a cut the old build refuses too), except: seed 94's final join was 1.7 mm³ too large before (the box adds 875 − 2.5 × 97.967 = 630.08 mm³ to 5270.48; now 5900.56, before 5902.27), and seed 71's hole differs only in the coarse measuring mesh (identical at 2e-4 mm). Every other volume change is the generator picking other edges by index for fillets and chamfers, some of which are now accepted; seed 71's join now builds.","seeds":[2,4,7,12,13,17,25,31,38,42,45,51,52,56,65,67,69,71,72,74,82,93,94,95]},
-  {"reason":"#1164: a pattern copy of a box sitting flush on the half-ring's top face (a 27 mm2 contact patch) now joins the ring instead of staying a separate shell, so the later random steps pick different edges and positions","seeds":[13]}
+  {"reason":"#1164: a pattern copy of a box sitting flush on the half-ring's top face (a 27 mm2 contact patch) now joins the ring instead of staying a separate shell, so the later random steps pick different edges and positions","seeds":[13]},
+  {"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]}
  ]
 }