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.

src/supervisor.rs

@@ -0,0 +1,37 @@
+//! Supervision signals.
+//!
+//! Every actor has a supervisor, which is itself just an actor with a
+//! `Receiver<Signal>`. When a child actor terminates, the scheduler sends
+//! a `Signal` on the supervisor's channel. The supervisor decides what to
+//! do — restart, escalate, ignore.
+//!
+//! For v0.1 there is no built-in restart-intensity cap. That's policy and
+//! lives in user code; library is mechanism only.
+
+use crate::pid::Pid;
+use std::any::Any;
+
+pub enum Signal {
+    /// The child exited normally.
+    Exit(Pid),
+    /// The child panicked. Payload is whatever `panic!` was called with.
+    Panic(Pid, Box<dyn Any + Send>),
+}
+
+impl std::fmt::Debug for Signal {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Signal::Exit(pid)     => write!(f, "Signal::Exit({:?})", pid),
+            Signal::Panic(pid, _) => write!(f, "Signal::Panic({:?}, ..)", pid),
+        }
+    }
+}
+
+impl Signal {
+    pub fn pid(&self) -> Pid {
+        match self {
+            Signal::Exit(p)     => *p,
+            Signal::Panic(p, _) => *p,
+        }
+    }
+}