requestAttention

Request the user's ATTENTION for the operation the CURRENT coroutine is inside (#264): emits an OperationAttention a presentation edge turns into a heads-up notification, to pull a user who left the app back to a step that needs them. Tagged with the op's OperationDescriptor.contentIntent so the alert taps back to the right screen. No-op outside a tracked operation. The edge decides whether to actually alert (e.g. suppress when foreground), so callers may fire this unconditionally.