Application-level shared DynamicSupervisor for ephemeral activity executor Tasks. Each child wraps Hourglass.ActivityRunner.run/3 (called via the 2-arg form, which fills in the default complete_fn arg pointing at BridgeHolder.complete_activity_task/2).

Started once for the whole VM (registered as __MODULE__). Promoted from a per-Worker DynSup to a shared one for the same reason as WorkflowEvaluator.DynamicSupervisor — activity executors have no per-task-queue state; they call BridgeHolder.complete_activity_task/2 with the task queue at completion time.

In-flight executors survive Worker child crashes (Workers no longer parent the DynSup). If the BridgeHolder restarts mid-execution the completion call returns {:error, :worker_not_registered} and Core redelivers via heartbeat-/start-to-close-timeout. Idempotency on activity-task redelivery is the caller's responsibility — Hourglass provides no built-in deduplication cache.

restart: :temporary — activity completion (success or failure) is shipped back to Core inside run/3, so OTP-level restart is not appropriate. Core owns activity-level retry.