perf(event): avoid locking manual-reset state reads and ready waits

[Kaileshwar16]

Summary

Let ManualResetEvent::is_set, try_wait, and already-set waits complete without taking
the waiter mutex. Keep the public API and existing waiter storage, with tests covering
registration races, rapid set/reset, cancellation, reentrant callbacks, and memory
publication.

Local benchmarks show single-thread state reads improving from 13.09 to 1.04 ns and
already-set waits from 15.03 to 5.83 ns. Replacing a pending waker costs about 28% more
because cloning outside the lock requires another lock acquisition. The benchmark report
includes methodology and tradeoffs.

Design Notes

State transitions and waiter registration remain serialized by the same mutex. Atomic
reads use Acquire and set publishes with Release. Released waiters retain their commitment
after reset.

Validation: all 597 tests passed, along with Clippy, formatting, documentation, event-
feature checks, and benchmark runs.

Related to #252; references #315. Waiter-storage replacement remains a separate
experiment.

[tisonkun]

We don't set a similar document for other benchmarks.

Typically, I'd prefer to make the benchmark suite self-described, or you may put necessary comments inline.