feat(monitor): unidirectional one-shot process monitors

`monitor(pid) -> Receiver<Down>` lets any actor watch any other and
receive a single Down{pid, reason} when it terminates. Generalizes the
existing single supervisor_channel (which is just a hard-wired monitor
the parent installs at spawn).

- src/monitor.rs: Down / DownReason{Exit,Panic,NoProc} + monitor().
  Payload-free: a panic payload has one owner and goes to the joiner via
  JoinError, so monitors learn only that it panicked. Monitoring an
  already-gone pid yields NoProc immediately.
- runtime: Slot grows `monitors: Vec<Sender<Down>>`; finalize_actor
  drains and notifies them outside the shared lock (send can unpark a
  receiver, which re-takes the non-reentrant mutex). Cleared in
  reclaim_slot and on slot reuse at spawn.
- Registration and finalize both serialize on the shared mutex, so a
  target alive at registration always delivers a real Down (no race
  between liveness check and registration).

tests/monitor.rs: Exit, Panic, NoProc-on-dead-target, and fan-out to
multiple monitors. Full suite green.

src/lib.rs

@@ -22,6 +22,7 @@ pub mod supervisor;
 pub mod timer;
 pub mod io;
 pub mod mutex;
+pub mod monitor;
 pub mod runtime;
 pub mod trace;
 
@@ -37,6 +38,7 @@ static ALLOCATOR: preempt::PreemptingAllocator = preempt::PreemptingAllocator;
 // ---------------------------------------------------------------------------
 
 pub use channel::{channel, Receiver, RecvError, Sender};
+pub use monitor::{monitor, Down, DownReason};
 pub use mutex::{LockTimeout, Mutex, MutexGuard};
 pub use pid::Pid;
 pub use runtime::{init, Config, Runtime};