Mesh: the tet mesher divides a seeded edge at the seed's nodes

The seed's spacing becomes size-field samples the field grades from, and
after coarsening its nodes are put on the feature curve and pinned, so the
edge has exactly the seeded divisions at the seeded spacing. The quality
report gives, per seed, the boundary edges along it.

Part of #230.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/mesh/src/seeds.rs

@@ -67,6 +67,11 @@ fn point_segment(p: P, a: P, b: P) -> f64 {
     dist(p, lerp(a, b, t))
 }
 
+/// The distance from `p` to a polyline.
+pub(crate) fn distance_to(poly: &[P], p: P) -> f64 {
+    point_polyline(p, poly)
+}
+
 fn point_polyline(p: P, poly: &[P]) -> f64 {
     match poly {
         [] => f64::INFINITY,