Started
Sealed parent of every "the run has started" event.
Concrete variants — ReplicationRunStarted, ScenarioRunStarted, ExperimentRunStarted, OptimizationRunStarted — carry a count field appropriate to their execution mode, but all share runId, modelIdentifier, and startTime. GUIs that only need a uniform "the run is now running" signal can match is RunEvent.Started without distinguishing the variant.