Make snapshots work even if the task DAG changes

Currently snapshots are associated with tasks and node snapshot data is keyed by the node ID from the DAG. These node IDs can easily change. We need a way to make saving/loading a snapshot work even if the DAG changes.

Start by tracing how snapshots are associated with tasks and how node snapshot data is keyed by DAG node IDs. Determine what stable association is needed for saving and loading when the DAG changes, then verify the behavior across changed DAGs. The issue does not name specific files, tests, or entry points.

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