feat: move TimerWheel from core to std via WorkerExtension
Add WorkerExtension trait (on_tick, handle_request, gc_dead) as a
per-worker counterpart to the shared RuntimeExtension. This enables
per-worker state like timer wheels to live outside core.
- TimerWheel, TimerRequest, CloneMsg → crates/std/src/timer_wheel.rs
- CtxTimers extension trait replaces Ctx::send_after_ticks/send_interval_ticks
- schedule_timer → generic post_worker_request on ContextInner
- Worker.timers → Worker.worker_ext (Option<Box<dyn WorkerExtension>>)
- StdExtension factory creates TimerWheel per worker
Core: 2440→2320 (−120). worker.rs: 696→599 (−97).
Cumulative: 2636→2320 (−316, −12.0%). worker.rs: 845→599 (−29.1%).
Authored by Claude, lovingly guided by Zachery Aaron Shores-Chmielewski
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