v0.1: green-thread actors, supervision, channels, benchmark

Hand-rolled context switching on mmap'd stacks with guard pages,
allocator-driven RDTSC preemption, unbounded MPSC channels, supervision
via per-slot Signal mailboxes, root supervisor as sentinel PID.

Lib + tests + benches clean check/clippy. All 29 tests pass.
Bench: smarm 3.4% over serial baseline, within 160us of tokio
current-thread on prime-counting fan-out.

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);
+}