refactor: centralize runtime logic (v0.4)

Extract scheduler responsibilities into a dedicated Runtime component:
- src/runtime.rs: New centralized control flow (669 lines)
- src/scheduler.rs: Simplified to task queue & preemption management
- tests/runtime.rs: Comprehensive runtime test suite
- benches/multi_scheduler.rs: Multi-runtime scheduling benchmarks
- Improves modularity and enables per-runtime configuration

src/timer.rs

@@ -28,7 +28,7 @@
 use crate::pid::Pid;
 use std::cmp::Reverse;
 use std::collections::BinaryHeap;
-use std::rc::Rc;
+use std::sync::Arc;
 use std::time::{Duration, Instant};
 
 /// What to do when a timer entry's deadline arrives.
@@ -45,7 +45,7 @@ pub enum Reason {
     /// target tell apart "this wait" from "a later wait by the same actor
     /// on the same target".
     WaitTimeout {
-        target: Rc<dyn TimerTarget>,
+        target: Arc<dyn TimerTarget>,
         wait_seq: u64,
     },
 }