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/lib.rs

@@ -0,0 +1,40 @@
+//! # smarm — Silly Marks Abstract Rust Machine
+//!
+//! Erlang-style green-thread actor concurrency for Rust.
+//!
+//! v0.1 is single-threaded. One scheduler, one OS thread. The scheduler
+//! cooperatively interleaves green-thread actors with hand-rolled context
+//! switches. Actors communicate by sending `Send` messages over channels;
+//! every actor has a supervisor, which is itself just an actor with a
+//! `Receiver<Signal>`.
+//!
+//! See `LOOM.md` for the design intent and the deferred-for-later list.
+
+pub mod stack;
+pub mod context;
+pub mod preempt;
+pub mod pid;
+pub mod actor;
+pub mod channel;
+pub mod scheduler;
+pub mod supervisor;
+
+// ---------------------------------------------------------------------------
+// Global allocator
+//
+// The preempting allocator wraps `System`. While `PREEMPTION_ENABLED` is
+// false (the default outside an actor) it adds one branch per allocation
+// and no syscalls. The scheduler flips it on per-resume.
+// ---------------------------------------------------------------------------
+
+#[global_allocator]
+static ALLOCATOR: preempt::PreemptingAllocator = preempt::PreemptingAllocator;
+
+// ---------------------------------------------------------------------------
+// Public API re-exports
+// ---------------------------------------------------------------------------
+
+pub use channel::{channel, Receiver, RecvError, Sender};
+pub use pid::Pid;
+pub use scheduler::{run, self_pid, spawn, spawn_under, yield_now, JoinError, JoinHandle};
+pub use supervisor::Signal;