feat: full runtime redesign (v0.6)

Complete rewrite with improved architecture & correctness:
- src/runtime.rs: Simplified task scheduling with proper state transitions
- src/scheduler.rs: Decoupled from runtime, pure task queue logic
- src/io.rs, src/mutex.rs: Refactored for clarity & performance
- New actor model framework (src/actor.rs, src/context.rs)
- Channel primitives (src/channel.rs) & process IDs (src/pid.rs)
- Preemption framework (src/preempt.rs) for fair timeslicing
- Expanded benchmarks & tests (multi_scheduler, primes, runtime)

tests/pid.rs

@@ -0,0 +1,22 @@
+use smarm::pid::Pid;
+
+#[test]
+fn pid_equality() {
+    assert_eq!(Pid::new(0, 0), Pid::new(0, 0));
+    assert_ne!(Pid::new(0, 0), Pid::new(0, 1));
+    assert_ne!(Pid::new(0, 0), Pid::new(1, 0));
+}
+
+#[test]
+fn pid_accessors() {
+    let p = Pid::new(42, 7);
+    assert_eq!(p.index(), 42);
+    assert_eq!(p.generation(), 7);
+}
+
+#[test]
+fn pid_debug_is_useful() {
+    let p = Pid::new(3, 5);
+    let s = format!("{:?}", p);
+    assert!(s.contains('3') && s.contains('5'), "got: {}", s);
+}