feat(link): bidirectional links + trap_exit (roadmap #3)

Add Erlang-style process links so an abnormal death fate-shares across a
link set, with trap_exit to convert that into a message instead.

- Slot.links: Vec<Pid>, bidirectional; reset in all three lifecycle sites.
- Actor.trap: Option<Sender<ExitSignal>>, fresh per spawn (a restarted
  child starts un-trapped; no fourth reset site).
- link/unlink free functions on self_pid(); trap_exit() -> Receiver<ExitSignal>
  (a dedicated inbox, distinct from the monitor Down channel).
- finalize_actor clears reverse links under the lock (always; keeps the
  cascade acyclic), then propagates abnormal deaths post-lock: trapping peer
  gets an ExitSignal message and survives, non-trapping peer is request_stop'd.
  Normal exit never propagates. Dead-pid link delivers an immediate NoProc
  (message if trapping, else request_stop(self) -- never a silent no-op).
- ExitSignal reuses DownReason and carries no panic payload (joiner-only).

Tests in tests/link.rs cover the propagate/trap/normal/dead-pid/unlink cases.

src/lib.rs

@@ -23,6 +23,7 @@ pub mod timer;
 pub mod io;
 pub mod mutex;
 pub mod monitor;
+pub mod link;
 pub mod runtime;
 pub mod trace;
 
@@ -38,6 +39,7 @@ static ALLOCATOR: preempt::PreemptingAllocator = preempt::PreemptingAllocator;
 // ---------------------------------------------------------------------------
 
 pub use channel::{channel, Receiver, RecvError, Sender};
+pub use link::{link, trap_exit, unlink, ExitSignal};
 pub use monitor::{monitor, Down, DownReason};
 pub use mutex::{LockTimeout, Mutex, MutexGuard};
 pub use pid::Pid;