Fillets and chamfers meeting at corners on curved faces: what #935 leaves (#854)

* Fillets and chamfers meeting at corners on curved faces, on top of #935

What #935 and #926 leave of #851 and the matrix:

- Corner build for blends meeting on faces of any surface (curvecorner.rs):
  the knife-edge tip's round with the arc's or the back edge's, edges
  picked in any order, a round ending in an earlier round of its radius
  (as a feature of its own), and chamfers meeting at the arc's corner.
  Its end curves are fitted by chord length: a uniform parameter
  through points clustered at the ends doubled back on itself there,
  and the display planner refused the face.
- A straight edge ending at a curved face (the back edge into the
  knife-edge tip): rolled, or cut by the face in the planar corner
  build (blend_corner.rs).
- A rolling ball turning about a rounded corner of its own radius
  (a pivot: a piece of sphere or cone).
- Seams that are an earlier blend's straight contact (fitted curves)
  count as straight.
- No spline across a joint where the strip's curvature jumps (a line
  into a small round): each strip ends on the joint's section.
- Sections at a chain's vertices on the faces of the edge from it
  alone.
- Every new fillet path passes #949's check for openings.

Tests measure volumes through the display plan, as #935's do.
Action matrix: 51 more fillets and chamfers pass (sweep, tangent arc
chain, sheet-metal hem, bracket holes), none fewer. Robustness seed 33
recorded (its last fillet builds differently; display and measured
volumes now agree).

Co-Authored-By: bherbruck <[email protected]>
Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_015bV2ntwLfUHV4JnCMPSyRi

* Corner blend tests: scale, seed 33's volumes, tighter engine bounds

From the review of #854:

- seed_33_display_plan_and_measure_agree: seed 33's bodies measure
  the same through the display plan and the measure (2e-5), which the
  golden's re-record rests on.
- the_arc_rounded_after_the_back_edge_matches_both_together: the order
  that still leaves 0.05 mm³ at the tip, as an ignored test.
- corner_blends_hold_at_any_scale: the cases this branch adds on the
  #851 part, at 0.01x and 100x, take off what they take at full size
  times k³ (within 5e-6, checked to 1e-4).
- The engine tests on the part as drawn measure through the display
  plan; bounds are about twice what each reads (2e-4 to 3e-3), and the
  front corner's sets are checked against the kernel's CSG grid.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_015bV2ntwLfUHV4JnCMPSyRi

---------

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

tests/robustness/goldens.json

@@ -34,7 +34,7 @@
   "30": {"area":853.525389,"bodies":1,"edges":23,"error":null,"faces":10,"outcome":"ok","rejected":0,"shells":1,"steps":["revolved section","chamfer"],"unsupported":0,"volume":1337.14821},
   "31": {"area":10195.46137,"bodies":3,"edges":69,"error":null,"faces":30,"outcome":"ok","rejected":1,"shells":3,"steps":["box","cut box","fillet","join box","circular pattern","cut box","circular pattern","hole"],"unsupported":1,"volume":56416.127527},
   "32": {"area":3220.802769,"bodies":1,"edges":51,"error":null,"faces":19,"outcome":"ok","rejected":0,"shells":1,"steps":["revolved section","cut sphere","cut box","cut box","join box"],"unsupported":0,"volume":7565.955897},
-  "33": {"area":5285.817318,"bodies":2,"edges":59,"error":null,"faces":27,"outcome":"ok","rejected":0,"shells":3,"steps":["revolved section","fillet","join box","cut cylinder","chamfer","join box","chamfer","fillet"],"unsupported":2,"volume":13859.24945},
+  "33": {"area":5286.736353,"bodies":2,"edges":59,"error":null,"faces":27,"outcome":"ok","rejected":0,"shells":3,"steps":["revolved section","fillet","join box","cut cylinder","chamfer","join box","chamfer","fillet"],"unsupported":2,"volume":13858.467983},
   "34": {"area":1086.303472,"bodies":1,"edges":30,"error":null,"faces":14,"outcome":"ok","rejected":0,"shells":1,"steps":["revolved section","cut sphere","hole"],"unsupported":0,"volume":2134.365275},
   "35": {"area":4605.630216,"bodies":1,"edges":26,"error":null,"faces":11,"outcome":"ok","rejected":0,"shells":1,"steps":["box","fillet","join box"],"unsupported":0,"volume":13965.526982},
   "36": {"area":10408.825748,"bodies":1,"edges":60,"error":null,"faces":26,"outcome":"ok","rejected":0,"shells":2,"steps":["revolved section","cut cylinder","join box","fillet","join cylinder"],"unsupported":0,"volume":55729.313103},
@@ -112,6 +112,7 @@
   {"reason":"the last-resort tool nudge is tried the other way when it fails or leaves sliver faces the operands didn't have: seed 43's join builds and the design runs to the end, 31 and 78 come out with fewer faces (#827, #897)","seeds":[31,43,78]},
   {"reason":"main's fillet changes since the last record (#926, #949) move seeds 33 and 91: a fillet step builds differently or is picked differently","seeds":[33,91]},
   {"reason":"hole tangent inside a stepped boss: touching sews split zero-thickness curved walls, so a pattern of holes spaced by their diameter (touching) in a stepped body cuts exactly instead of by a tool nudge: 42 builds more faces (the walls split where the holes touch), 51 picks other edges afterwards","seeds":[42,51]},
+  {"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 mm\u00b3 (area by 0.04 mm\u00b2)","seeds":[81]}
  ]
 }