Fix EndSink highway: skip group when no horizontal overlap exists

DetectEndSinkGroups was forming highways for edges at different Y
levels with NO shared corridor. The fallback (line 1585) used
min-MaxX as collector when overlap detection failed, creating a
false highway that truncated individual edge paths.

Fix: skip the group entirely when TryResolveHorizontalOverlapInterval
returns false. Edges at different Y levels render independently.

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

src/Workflow/__Libraries/StellaOps.Workflow.Renderer.Svg/WorkflowRenderSvgRenderer.cs

@@ -1582,8 +1582,9 @@ public sealed class WorkflowRenderSvgRenderer
                     out var corridorStartX,
                     out var collectorX))
             {
-                collectorX = corridorIntervals.Min(interval => interval.MaxX);
-                corridorStartX = corridorIntervals.Max(interval => interval.MinX);
+                // No shared horizontal overlap — don't form a highway.
+                // Edges at different Y levels should render independently.
+                continue;
             }
 
             if (collectorX <= corridorStartX + 18d)