Function observe_run_start 

pub fn observe_run_start(
    plan: &Plan,
    lineage: LineageId,
    writer: &ActorId,
    request: &RunStart,
    now: DateTime<Utc>,
) -> Result<RunRecord, RunError>

Validate a start against the plan and capture the contract it observes.

The task must be an executable task its executor already holds, claimed or started. writer is the principal recording the run: the executor itself, or a human or service acting for it. lineage is the history plan was read from; the record keeps it with the plan revision so the observation stays attributable after a restore. now is DPM’s own clock, the receipt time.