feat(cancel): cooperative cancellation via sentinel unwind

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.

src/lib.rs

@@ -43,8 +43,8 @@ pub use mutex::{LockTimeout, Mutex, MutexGuard};
 pub use pid::Pid;
 pub use runtime::{init, Config, Runtime};
 pub use scheduler::{
-    block_on_io, run, self_pid, sleep, spawn, spawn_under, wait_readable, wait_writable,
-    yield_now, JoinError, JoinHandle,
+    block_on_io, request_stop, run, self_pid, sleep, spawn, spawn_under, wait_readable,
+    wait_writable, yield_now, JoinError, JoinHandle,
 };
 pub use supervisor::{ChildSpec, OneForOne, Restart, Signal};