docs(rivet): track kiln-async external-wake + host-future/Waker bridge gaps

[avrabe]

Lands two kiln-async gaps as tracked rivet requirements (roadmap-only; no code). Surfaced by the executor-comparison benchmark (Findings 1 & 8) and the bench bridge doc comment — previously only phase notes under REQ_ASYNC_SCHED.

REQ_ASYNC_EXTWAKE — external wake-up path (v0.5.0)

Scheduler::mark_ready is public and TaskOutcome::Waited parks a task, but mark_ready is only ever driven by internal scheduler events inside poll_round. No out-of-loop event source (timer/ISR/host callback/gale event-group) can wake a parked task, so a task cannot today wait on a real external event and be resumed later. Needs a sound external-wake API preserving no_std / no-alloc / forbid(unsafe_code) / fuel-bounded, with an explicit concurrency model.

REQ_ASYNC_WAKERBRIDGE — official host Future/Waker bridge (v0.6.0)

kiln-async is #![forbid(unsafe_code)] permanently and deliberately never polls a Rust Future or builds a RawWaker vtable. The benchmark's bridge is explicitly unofficial/non-production — sound only for futures that self-wake synchronously inline (e.g. CooperativeYield), not general futures that store/clone the Waker. The "R1 unsafe waker" belongs in a separate std-only companion crate (may use unsafe) whose wake() routes to the external-wake path — outside the cert crate. Not a defect; a deliberately-descoped feature.

Both derives-from REQ_ASYNC_SCHED; the bridge depends-on REQ_ASYNC_EXTWAKE. rivet validate = 0 errors.

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