Function materialize 

pub async fn materialize(
    backend: &dyn StateBackend,
    execution_id: &ExecutionId,
) -> Result<MaterializedState, StateBackendError>

Reconstruct the current workflow state from the event log.

Algorithm:

1. Load the latest snapshot (if any). If none, start from the initial_input.
2. Load all events since the snapshot’s at_sequence.
3. Apply state patches from NodeCompleted events in order.
4. Derive status, completed_nodes, and active_nodes from all events.