fix: Convert joined signals Dict to List for NodeExecutionState

NodeExecutionState expects normalized_signals as List[NormalizedSignal],
but join_evaluator returns Dict[str, NormalizedSignal].

Fix: Convert dict to list before returning NodeExecutionState.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/workflow_executor.py

@@ -481,12 +481,16 @@ def execute_join_node(
             f"Join node {node.id}: Consolidated {executed_count}/{total_count} paths"
         )
 
-        # 5. Return NodeExecutionState
+        # 5. Convert consolidated_signals Dict → List[NormalizedSignal]
+        # (NodeExecutionState expects List, but join_evaluator returns Dict)
+        consolidated_signals_list = list(join_result.consolidated_signals.values())
+
+        # 6. Return NodeExecutionState
         return NodeExecutionState(
             node_id=node.id,
             status=NodeStatus.EXECUTED,
             analysis_core=consolidated_core_json,
-            normalized_signals=join_result.consolidated_signals,
+            normalized_signals=consolidated_signals_list,
             metadata=join_result.metadata,
             started_at=started_at,
             completed_at=datetime.utcnow().isoformat()