fix(elksharp): preserve source/target endpoints in corridor reroute clearance push

When the corridor reroute pushes a horizontal segment away from a
blocking node, preserve the first point (source connection) and
insert a vertical step to reconnect the last point (target connection)
at the original Y. Previously, pushing all points uniformly would
disconnect the edge from its target node when the push Y exceeded
the target node's boundary.

Fixes edge/9 (Retry Decision → Set batchGenerateFailed) which was
pushed to Y=653 but the target node bottom is at Y=614 — the endpoint
now steps back up to Y=592 to reconnect.

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

src/__Libraries/StellaOps.ElkSharp/ElkEdgeRouterIterative.BoundaryFirst.CorridorReroute.cs

@@ -209,12 +209,30 @@ internal static partial class ElkEdgeRouterIterative
                     && bestPushY < graphMaxY + 56d
                     && Math.Abs(bestPushY - laneY) > 2d)
                 {
-                    var newPath = new List<ElkPoint>(path.Count);
+                    var newPath = new List<ElkPoint>(path.Count + 2);
                     for (var i = 0; i < path.Count; i++)
                     {
                         if (i >= bestSegStart && i <= bestSegStart + 1
                             && Math.Abs(path[i].Y - laneY) <= 2d)
                         {
+                            // Preserve source connection (first point).
+                            if (i == 0)
+                            {
+                                newPath.Add(path[i]);
+                                continue;
+                            }
+
+                            // Preserve target connection (last point) —
+                            // insert a vertical step to reconnect.
+                            if (i == path.Count - 1)
+                            {
+                                var stepX = path[i].X + (path[i - 1].X > path[i].X ? 24d : -24d);
+                                newPath.Add(new ElkPoint { X = stepX, Y = bestPushY });
+                                newPath.Add(new ElkPoint { X = stepX, Y = path[i].Y });
+                                newPath.Add(path[i]);
+                                continue;
+                            }
+
                             newPath.Add(new ElkPoint { X = path[i].X, Y = bestPushY });
                         }
                         else