MilanoActionHandler

An asynchronous receiver of custom actions: one funnel per view. Normal return is success and the returned value, validated against the action's declared result type, binds the result root inside onSuccess; return null for actions declaring no result. Throwing is failure: a MilanoActionFailure carries the failure payload, validated against the declared failure type and bound to the failure root inside onFailure; any other exception is a failure with no payload. Completion-exactly-once holds by construction.