Use Task.Run for ignored Action.BeginInvoke calls

Opt-in port of resolved System.Action.BeginInvoke(null, null) expression statements. Require a fresh Action construction or a closed local initialized by construction, lambda or an unqualified in-class method group. Retain immediate receiver evaluation and delegate capture. Adopt Task.Run scheduling, child-task and unobserved-exception behavior; this recipe is not included in version composites. Leave callbacks/state, observed results, named arguments, nullable/factory/escaped/reassigned delegates, other delegate types, UI dispatchers, expression trees, discarded comments and conditional snapshots unchanged.