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 <brennen.herbruck@gmail.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015bV2ntwLfUHV4JnCMPSyRi

---------

Co-authored-by: bherbruck <brennen.herbruck@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/kernel/src/loopblend.rs

@@ -271,6 +271,27 @@ fn build(
         side = sd;
         walls.push(wf);
     }
+    // A run ends in the cross-section through its end vertex: the face there must hold it (a
+    // plane square to the run). One at an angle cuts the blend's two contacts at different
+    // places, which the rolling-ball blend handles.
+    if open {
+        let ends = [
+            (edges.first().map(|e| e.front().clone()), segs.first().map(|s| (s.start(), *s))),
+            (edges.last().map(|e| e.back().clone()), segs.last().map(|s| (s.end(), *s))),
+        ];
+        for (v, seg) in ends {
+            let (Some(v), Some((p, s))) = (v, seg) else { return Err(KernelError::Failed("run end".into())) };
+            let t = s.tangent(p).ok_or_else(|| unsupported("degenerate loop edge"))?;
+            let square = faces
+                .iter()
+                .enumerate()
+                .filter(|(gi, g)| *gi != fi && !walls.contains(gi) && g.vertex_iter().any(|x| x == v))
+                .all(|(_, g)| plane_normal(g).is_some_and(|gn| gn.dot(t).abs() > 1.0 - 1e-6));
+            if !square {
+                return Err(unsupported("a run of edges ending at a face at an angle"));
+            }
+        }
+    }
     // Cross-section at a loop point: into the face (m) and up the wall (t, away from the edge,
     // up a concave wall and down a convex one). The ball of radius r touches both at the
     // setback s = r / tan(φ/2), φ the angle between m and t; its centre is on the bisector.