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/pid.rs

@@ -0,0 +1,38 @@
+//! Process identifiers.
+//!
+//! A `Pid` is `(index, generation)`. The index is a slot in the scheduler's
+//! actor table; the generation increments every time that slot is reused.
+//! A stale `Pid` (correct index, wrong generation) is a detectable error,
+//! not a silent misdirection — solves the ABA problem without exhausting
+//! the PID space.
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash)]
+pub struct Pid {
+    index: u32,
+    generation: u32,
+}
+
+impl Pid {
+    #[inline]
+    pub const fn new(index: u32, generation: u32) -> Self {
+        Self { index, generation }
+    }
+
+    #[inline]
+    pub const fn index(self) -> u32 { self.index }
+
+    #[inline]
+    pub const fn generation(self) -> u32 { self.generation }
+}
+
+impl std::fmt::Debug for Pid {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "Pid({}.{})", self.index, self.generation)
+    }
+}
+
+impl std::fmt::Display for Pid {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "<{}.{}>", self.index, self.generation)
+    }
+}