Add actor lifecycle hooks and two-mode graceful stop mechanism:
- ActorInterface::on_start() called once before first message (panic = poison)
- ActorInterface::on_stop() called on graceful stop (NOT on panic - unsafe)
- ctx.stop_self() for immediate self-stop after current message
- runtime.stop_actor() for external stop (PoisonPill semantics, queued)
- Separate stats tracking: stops counter distinct from panics
- 12 new behavioral tests, 82 total passing
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Research across ractor, tokio, Erlang/OTP BEAM, Linux CFS, and libuv
revealed that tick_all drained the entire mailbox per actor per tick,
allowing one hot actor to starve all others on the same worker.
- Add `actor_message_budget` to RuntimeConfig (default: 64 msgs/actor/tick)
- Modify tick_all to break after budget messages, yielding to next actor
- budget=0 restores unlimited (backward compatible) behavior
- 3 new fairness tests validating hot-cold actor scenarios
- New fairness benchmark group (cold_latency_under_pressure, throughput_by_budget)
- Fix RuntimeConfig struct literals across workspace crates
Inspired by BEAM's 4000-reduction budget and tokio's 128-op cooperative budget.
All 45 tests pass (42 original + 3 new).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
