fix: straight edge routing and handler section edge extraction

Backend:
- Set POLYLINE edge routing on ELK root — eliminates curved/bent edges
  between horizontally aligned nodes
- Collect edges from handler section roots (not just main root) so
  internal handler edges are included in the layout output
- Use correct root reference for coordinate calculation per edge

Frontend:
- Render ALL edge points as line segments (polylines), not cubic bezier.
  ELK bend points are waypoints, not bezier control points — the cubic
  bezier interpretation caused false curves.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/ProcessDiagram/DiagramEdge.tsx

@@ -11,19 +11,11 @@ export function DiagramEdge({ edge, offsetY = 0, traversed }: DiagramEdgeProps)
   const pts = edge.points;
   if (!pts || pts.length < 2) return null;
 
-  // Build SVG path: move to first point, then cubic bezier or line to rest
+  // Build SVG path: move to first point, then line segments through all waypoints.
+  // ELK bend points are waypoints, not bezier control points.
   let d = `M ${pts[0][0]} ${pts[0][1] + offsetY}`;
-
-  if (pts.length === 2) {
-    d += ` L ${pts[1][0]} ${pts[1][1] + offsetY}`;
-  } else if (pts.length === 4) {
-    // 4 points: start, control1, control2, end → cubic bezier
-    d += ` C ${pts[1][0]} ${pts[1][1] + offsetY}, ${pts[2][0]} ${pts[2][1] + offsetY}, ${pts[3][0]} ${pts[3][1] + offsetY}`;
-  } else {
-    // Multiple points: connect with line segments through intermediate points
-    for (let i = 1; i < pts.length; i++) {
-      d += ` L ${pts[i][0]} ${pts[i][1] + offsetY}`;
-    }
+  for (let i = 1; i < pts.length; i++) {
+    d += ` L ${pts[i][0]} ${pts[i][1] + offsetY}`;
   }
 
   return (