feat: lay out PARALLEL branches as vertical columns

Co-authored-by: Cursor <cursoragent@cursor.com>

src/stores/designer.ts

@@ -252,8 +252,9 @@ export const useDesignerStore = defineStore('designer', () => {
     publishedVersion.value = params.version ?? null
     loadedFromDraft.value = Boolean(params.fromDraft)
     const expanded = expandLegacyParallel(params.nodes, params.edges)
-    nodes.value = fitParallelFrames(expanded.nodes)
-    edges.value = assignEdgeHandles(expanded.nodes, expanded.edges)
+    const parallels = expanded.nodes.filter((node) => node.data?.kind === 'PARALLEL').map((node) => node.id)
+    nodes.value = parallels.reduce((current, id) => reflowLanes(current, id), expanded.nodes)
+    edges.value = assignEdgeHandles(nodes.value, expanded.edges)
     syncStartFlags()
     clearSelection()
     resetHistory()
@@ -707,10 +708,10 @@ export const useDesignerStore = defineStore('designer', () => {
     pushHistory()
     const laneIndex = parent.data.lanes.indexOf(lane)
     const stepIndex = nodes.value.filter((node) => node.parentNode === parallelId && laneOf(node.data) === lane).length
-    nodes.value = fitParallelFrames([
-      ...nodes.value,
-      spawnLaneStep(parallelId, lane, laneIndex, stepIndex, nextId(lane), kind),
-    ])
+    nodes.value = reflowLanes(
+      [...nodes.value, spawnLaneStep(parallelId, lane, laneIndex, stepIndex, nextId(lane), kind)],
+      parallelId,
+    )
   }
 
   function renameLane(parallelId: string, from: string, to: string): boolean {