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>
Adds some basic benchmarking, stress tests. They still need to be properly examined to ensure they are testing the correct properties, but fit for "good enough". Implements the HybridChannel type, which features a channel buffer that can withstand overflows. It does so by providing a dequeue behind a mutex. Without overflow, will push messages into the lock free ArrayQueue implemented by crossbeam_queue; when that buffer fills, will use the locking portion provided by the Mutex<VecDequeue>.
In the future we can even further optimize this, perhaps with some linked list implementations of lock-free channels, but, like the benchmarks, this fits the "good enough" bar for now.
