Attempt to produce this computation's value. Returns the completed result if it is ready; otherwise registers onComplete (to be invoked when the value becomes available) and returns a still-pending Async.
A still-pending result is either this pollable itself (the common case) or a '''replacement''' pollable that represents the rest of the computation; the caller must direct its next poll at whatever pollable was returned. The built-in drivers (block, start, the interop runners) and every combinator follow this, so a conforming poll may hand back a new pollable at any suspension.
Returning the same identity means the caller may wait for the registered callback. Returning a distinct identity is synchronous progress: the caller must poll that replacement without waiting. An identity change is not a readiness event, so a caller or wrapper must not synthesize onComplete for it. Real callbacks may be stale, reentrant, or duplicated and are therefore coalescible wake permits rather than proof that a particular generation has completed.
Polling is a '''one-shot driver protocol''': a driver should keep polling only while poll returns a still-pending Pollable, and must stop as soon as it returns a terminal result — a raw value or a failed Async. The built-in drivers (block, start, and the interop runners) all follow this, so they never poll a settled computation again.
Re-polling a Pollable after it has returned a terminal result is outside the contract and is '''not''' guaranteed to be a pure re-observation: a continuation may run again and diagnostic state may be repeated. Drive computations through the built-in runners unless you are implementing this protocol yourself, in which case honor the stop-at-terminal rule.
