Loop blends: accept straight wall seams that are not lines (#335)

* Loop blends accept straight wall seams that are not lines

Filleting or chamfering the top outline of two overlapping cylinders whose
concave edges were rounded failed with "the wall seams must be straight":
the round's contact lines with the cylinders are straight NURBS curves, and
the loop blend only accepted seams stored as lines. A seam now counts as
straight when every sample lies on its chord; it is trimmed as a line as
before.

* Fillet and chamfer arrows point into the body

The radius arrow pointed out of the body along the edge's bisector, so
dragging it away from the edge grew the round. It now points into the
material, like the shell's thickness arrow: dragging toward the body grows
the radius or distance, dragging out of it shrinks it.

crates/kernel/src/loopblend.rs

@@ -97,6 +97,22 @@ fn seg_of(e: &mt::Edge, tol: f64) -> Option<Seg> {
     Some(Seg::Arc { a, c: centre, axis, angle, mid: m })
 }
 
+/// Whether an edge is a straight segment: a line, or a curve whose points all lie on the chord
+/// (the contact lines of a rolling-ball blend between parallel walls are such curves).
+fn is_straight(e: &mt::Edge, tol: f64) -> bool {
+    use mt::{BoundedCurve, ParametricCurve};
+    if matches!(e.curve(), mt::Curve::Line(_)) {
+        return true;
+    }
+    let (a, b) = (from_p3(e.absolute_front().point()), from_p3(e.absolute_back().point()));
+    if a.dist(b) < tol {
+        return false;
+    }
+    let c = e.curve();
+    let (t0, t1) = c.range_tuple();
+    (0..=16).all(|k| from_p3(c.subs(t0 + (t1 - t0) * k as f64 / 16.0)).dist_to_segment(a, b) < tol)
+}
+
 /// `p` turned about the line (c, axis) by `ang`.
 fn turn(p: Vec3, c: Vec3, axis: Vec3, ang: f64) -> Vec3 {
     let k = axis.normalized().unwrap_or(Vec3::Z);
@@ -518,7 +534,7 @@ fn build(
                 }
                 let (fa, fb) = (e.absolute_front().clone(), e.absolute_back().clone());
                 let other = if &fa == v { &fb } else { &fa };
-                if !matches!(e.curve(), mt::Curve::Line(_)) {
+                if !is_straight(&e, (size * 1e-5).max(1e-6)) {
                     return Err(unsupported("the wall seams must be straight"));
                 }
                 let face_side = on_face(&e);