fix: inner_join must explicitly store inherited label for outer_join access

In subprocess execution (Dagster), each task's artifact file only contains
attributes explicitly assigned during that step — lazy inheritance is not
persisted. Without self.label = self.label in inner_join, the outer_join
step cannot read label from the inner_join task artifact, causing
AttributeError: Flow SplitInBranchFlow has no attribute 'label'.

Author: Nissan Pow

test/ux/core/flows/basic/split_in_branch_flow.py

@@ -35,6 +35,11 @@ def inner_y(self):
     @step
     def inner_join(self, inputs):
         self.sub_labels = sorted(i.sub_label for i in inputs)
+        # Explicitly read-and-store inherited 'label' so that outer_join can
+        # access it from this task's artifact file in subprocess execution.
+        # Without this, label is lazily inherited during execution but never
+        # persisted to the artifact store, causing AttributeError downstream.
+        self.label = self.label
         self.next(self.outer_join)
 
     @step