Add Future completion subscriptions

[simPod]

Context

Promise adapters may need to observe a Future completion without transforming it into another Future.

Decision

Add Future::subscribe() and Future::unsubscribe() as public facades for the existing one-shot completion callback mechanism.

Consequences

Adapters can observe values and errors without allocating derived Future chains. Callback subscriptions can be removed before a Future completes.

Benchmark Context

A wide asynchronous result graph with 838 parent objects and three Future-backed fields measured the cost of moving from Fiber-per-continuation handling to direct callback composition:

Adapter strategy	CPU time	Peak memory
Fiber per continuation	2.11 s	274 MB
Direct callback composition	1.67-1.70 s	149 MB

That workload reduced CPU time by about 19% and peak memory by about 46%. These measurements provide motivation for a public completion subscription API; they are not a general performance guarantee.

[simPod]

Superseded by upstream PR amphp#469.