fix(library): keep a bend lane the user dragged close to a stub, don't snap it clear

Root cause of diagram 87's "drag jumps back": after the release preserved the dragged
lane, the final `pushLanesClearOfStubs` pass enforced a 30px clearance between any lane
and a parallel stub and shoved the lane straight back to the clearance line — so dragging
the down-run of a narrow U to 15px from the up-stub snapped it from 665 back to 650.

That clearance exists to keep a FRESH route from stair-stepping through a sliver next to a
stub. It must not override geometry the user placed: a lane the stored/dragged route
already has on THIS side of the stub is theirs, even a deliberately narrow U. Push now
fires only for a lane with no stored side yet, or one that FLIPPED across the stub to the
far side; a same-side stored lane is kept.

diagram 87: the dragged lane now sticks at 665 (was snapping to 650); dragging it all the
way onto the stub still folds and collapses the U. Unit test covers the narrow-U lane;
1527 unit + quality gate, node-move re-projection, fresh-bend, interactions, reset,
anchoring, live-reflow e2e green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: FelixTJDietrich

library/lib/utils/edgeUtils.ts

@@ -1999,7 +1999,12 @@ const pushLanesClearOfStubs = (
           previous !== line &&
           Math.sign(start[lane] - line) !== Math.sign(previous - line)
       )
-    if (isClear(start[lane]) && !flipped) continue
+    // A lane the stored route already placed on THIS side of the stub is the user's —
+    // even a deliberately narrow U that runs close to a parallel stub. Keep it. Only
+    // re-clear a lane that has no stored side yet (a fresh route) or one that FLIPPED
+    // across the stub to the far side. Otherwise dragging a lane toward a stub would be
+    // snapped straight back to the clearance line.
+    if (!flipped && (isClear(start[lane]) || previous !== undefined)) continue
 
     const reference = previous ?? start[lane]
     const candidates = lines

library/tests/unit/edgeUtils.test.ts

@@ -840,6 +840,27 @@ describe("removeDuplicatePoints", () => {
 // preserveOrthogonalEdgePoints
 // ---------------------------------------------------------------------------
 describe("preserveOrthogonalEdgePoints", () => {
+  it("keeps a lane the user dragged close to a parallel stub (a narrow U), not snapped clear", () => {
+    // diagram 87: the down-run (x=665) was dragged to 15px from the source up-stub
+    // (x=680). It must STAY at 665 — pushing it to the 30px clearance line (650) snapped
+    // the drag straight back. A lane already on this side of the stub is the user's.
+    const dragged = [
+      { x: 680, y: 155 },
+      { x: 680, y: -150 },
+      { x: 665, y: -150 },
+      { x: 665, y: -10 },
+      { x: 575, y: -10 },
+    ]
+    const result = preserveOrthogonalEdgePoints(
+      dragged,
+      { x: 680, y: 155 },
+      { x: 575, y: -10 },
+      Position.Top,
+      Position.Right
+    )
+    expect(result).toEqual(dragged)
+  })
+
   it("keeps a manually moved bend when the target handle moves lower on the same side", () => {
     const result = preserveOrthogonalEdgePoints(
       [