Add watch/unwatch API to the actor system so actors can monitor each
other's liveness. When a watched actor dies (panic or stop), watchers
receive an ActorExited notification via on_actor_exit().
- ExitReason enum (Stopped, Panicked, NodeDown) and ActorExited struct
- ContextInner::watch()/unwatch() + Ctx typed wrappers
- ActorInterface::on_actor_exit() default method (system message fallback)
- WatchRegistry in worker with bidirectional tracking
- Death notification dispatch as phase 5b in tick_once
- Runtime-level watch for external callers
- 10 behavioral tests in tests/watch_api.rs
- Design documents for OS features in docs/os-design/
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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