Poll operation

The engine now exposes Poll(ctx, flowKey) alongside Await. Like Await, it blocks up to the context deadline for a flow to stop, but where Await treats a deadline expiring before the flow stops as a timeout error, Poll instead returns the flow's current outcome — non-terminal, with Stopped() reporting false — and no error. This makes it the natural fit for bridging an open-ended durable flow to a bounded request (for example, answering an HTTP long-poll within its time budget and re-polling on the next request) without conflating "not done yet" with a real failure. The returned FlowOutcome gains a Stopped() helper to distinguish a terminal/interrupted outcome from one still running. As always, the underlying flow keeps running on the engine independent of the caller; a ctx timeout never tears it down.