Add three features to enable Erlang-style supervision:
1. ActorInterface::handle_down() — callback for monitored actor deaths,
allowing actors to react to Down messages without making Down their
Incoming type. Backward-compatible: actors with Incoming=Down still
receive through handle().
2. ctx.stop_actor(addr) — send graceful stop to another actor from
handler context using PoisonPill semantics.
3. Supervisor actor — manages child actors with configurable restart
policies (Permanent/Transient/Temporary), OneForOne strategy, and
meltdown detection (max_restarts). Built entirely on existing
primitives (monitor, spawn, Down, lifecycle hooks).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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