fix: a lenient parameter boundary found across a seam is not used; record seed 81 (#1003)

#970's lenient retry of a face's parameter boundary searches the whole surface as a last resort; on robustness seed 20 it found an edge's points across a cylinder's seam, half a turn apart, and the Cut failed. A lenient boundary whose points jump half the parameter range or more is refused, and the face is left undivided as before (#970's elbows step at most a fifth of the range). Seed 81's volume moved by 0.1 mm3 with #952's hole direction; recorded.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/tests/robustness.rs

@@ -549,3 +549,13 @@ fn blend_corners_where_edges_meet_on_a_curved_wall() {
     let removed = base - planned_volume(&s);
     assert!((removed - 67.05).abs() < 0.15, "three at the corner: removed {removed}, expected 67.05");
 }
+
+/// Seed 20 cuts a patterned cylinder's holes through a filleted plate. The lenient retry of a
+/// face's parameter boundary (#970) searched the whole cylinder for a point and found it across
+/// the seam, half a turn away; the face was divided wrongly and the cut failed. Such a boundary
+/// is refused now, and the cut builds as before #970.
+#[test]
+fn a_boundary_found_across_a_seam_is_not_used() {
+    let r = fuzz::run(20, 8);
+    assert_eq!(r.outcome.kind, "ok", "seed 20: {:?}\n{}", r.outcome, r.script());
+}