Add link and monitor operations for actors

[ElFantasma]

Erlang-style process relationships, adapted to Spawned's type system (no Pids — uses ActorRef<A> and Recipient<M>).

Linking (bidirectional):

• When a linked actor stops, the other is notified (and stopped, unless trapping exits)
• start_linked() — start an actor linked to the caller

Monitoring (one-way):

• Observer receives a notification when the monitored actor stops
• monitor() → MonitorRef
• demonitor(monitor_ref)

Exit trapping:

• trap_exit() — convert linked exit signals into messages instead of stopping

Design notes:

• No Pid type — link/monitor targets are identified via ActorRef or Recipient
• Exit notifications delivered as messages to the actor's handler (exact message type TBD during implementation)
• Foundation for supervision (Add Supervisor actor for fault tolerance #133, Add DynamicSupervisor for runtime child creation #134)

Part of Phase 3 (Supervision Trees) — see docs/ROADMAP.md.