How to use `Events` together with async event loops

In async event loops like `amphp`, the main thread can sleep until a specific condition is satisfied, such as a stream is available for read/write, a child process is exited, or a time interval has passed. However, when I try to use ext-parallel inside an amphp-based program, I found that I have do busy polling with current API of `Events`, which uses a lot of CPU time. Is there anything I missed that can let me trigger an action _on the main thread asynchorously_ when an event is ready?

The issue names the Events API and amphp-based event loops but no files or tests. Start by reading the Events API and tracing how event readiness is currently polled in an amphp integration. Done means identifying a supported main-thread wake-up mechanism or clearly documenting that the current API cannot provide one.

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