fix(integrate): sort boundary edges as pairs

Assisted-by: GLM 5.3 Flash

integrate/fem2d.go

@@ -114,13 +114,25 @@ func (m *TriangleMesh2D) BoundaryEdges() []int {
 		count[key(b, c)]++
 		count[key(c, a)]++
 	}
-	edges := make([]int, 0, 8)
+	sets := make([][2]int, 0, len(count))
 	for e, n := range count {
 		if n == 1 {
-			edges = append(edges, e[0], e[1])
+			sets = append(sets, e)
 		}
 	}
-	slices.Sort(edges)
+	// The pairs sort as pairs, never as one flat index list: a flat sort
+	// interleaves the endpoints of unrelated edges and hands back pairs
+	// the mesh does not carry.
+	slices.SortFunc(sets, func(x, y [2]int) int {
+		if x[0] != y[0] {
+			return x[0] - y[0]
+		}
+		return x[1] - y[1]
+	})
+	edges := make([]int, 0, 2*len(sets))
+	for _, e := range sets {
+		edges = append(edges, e[0], e[1])
+	}
 	return edges
 }