request_stop(pid) sets a per-actor flag and wakes a parked target. The
actor realizes the stop as a controlled unwind at its next observation
point (check!()/alloc via maybe_preempt, or the wakeup side of any
blocking park): a StopSentinel panic tears the stack down via the
trampoline's existing catch_unwind, running Drop, and is reported as the
new Outcome::Stopped (distinct from a user Panic).
Death surface kept distinct from Exit: Signal::Stopped(pid) +
DownReason::Stopped, so the supervisor await logic to come can tell a
requested stop apart from a self-termination.
Flag lives on Actor behind Arc<AtomicBool>; the scheduler resume path
takes a raw pointer into it (no per-resume refcount traffic), keeping
yield throughput at baseline.
