Wrong stats in multi-node local executor

Due to lack of node-level communication, the stats at the end of each pipeline step can only aggregate results from the current node, and what's being written to the disk is the status of the last finished worker, rather than the global info

Start by locating the multi-node local executor and the code that aggregates pipeline-step stats and writes status to disk. Trace how worker results are collected, then verify that the persisted output represents global node information rather than the last finished worker.

Written by the indexing model from the issue text.