feat(channel,runtime): off-runtime cross-thread wake for parked receivers and stop

Wakes issued from a non-scheduler OS thread were silent no-ops. Every
off-runtime wake primitive (unpark, unpark_at, request_stop) reaches the
runtime through the RUNTIME thread-local, which is unset on any foreign
thread — so a send from a plain std::thread enqueued its message but never
woke the parked receiver, and there was no way to drive a stop into a
runtime from an application thread (e.g. an OS-signal handler). The former
strands a parked recv forever; the latter is why a downstream server must
poll a shutdown flag instead of parking on it.

Generalize RFC 018's rule — a producer reaches the runtime through a Weak it
holds — from the IO backend to channel senders and to a new handle:

- A receiver captures a Weak<RuntimeInner> (provably live at that moment)
  alongside its (pid, epoch) when it parks. send() and the last-sender drop
  wake through scheduler::unpark_at_via, which takes the thread-local path
  when on a scheduler thread (preemption-gated, slot-eligible) and the
  captured Weak otherwise — the same cross-context wake the IO threads do.
- Runtime::handle() returns a Send + Sync RuntimeHandle carrying that Weak;
  RuntimeHandle::request_stop drives a cooperative stop from any thread and
  is a no-op once the runtime is dropped.

The in-runtime wake paths (recv/select timers) are unchanged; only the sites
reachable from a foreign thread route through the Weak. RuntimeHandle exposes
request_stop only: send-wake needs no user-facing handle, and off-runtime
unpark is covered because request_stop drives unpark on the upgraded inner.

tests/cross_thread_wake.rs: a foreign-thread send wakes a parked receiver; a
foreign-thread request_stop wakes and stops a parked actor; a RuntimeHandle
held across and beyond run never blocks all-done and degrades to a no-op.

src/lib.rs

@@ -85,7 +85,7 @@ pub use registry::{
     install, lookup_as, register, resolve_name, send, send_dyn, send_to, unregister, whereis,
     NameResolution, RegisterError, SendError,
 };
-pub use runtime::{init, Config, Runtime};
+pub use runtime::{init, Config, Runtime, RuntimeHandle};
 pub use scheduler::{
     block_on_io, cancel_timer, request_stop, run, self_pid, send_after, send_after_named,
     send_after_named_wall, send_after_wall, sleep, sleep_wall, spawn, spawn_addr, spawn_addr_with,