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)

src/lib.rs

@@ -0,0 +1,60 @@
+//! # smarm — Silly Marks Abstract Rust Machine
+//!
+//! Erlang-style green-thread actor concurrency for Rust.
+//!
+//! Multi-threaded: N scheduler OS threads (default: one per CPU) share a
+//! single global run queue behind a `Mutex`. Actors communicate by sending
+//! `Send` messages over channels; every actor has a supervisor. Synchronisation
+//! primitives — `Mutex<T>` with mandatory lock timeouts, channel `recv`,
+//! `sleep`, and epoll-backed `wait_readable`/`wait_writable` — all park the
+//! green thread, never the OS thread.
+//!
+//! 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;
+pub mod timer;
+pub mod io;
+pub mod mutex;
+pub mod runtime;
+pub mod trace;
+
+// ---------------------------------------------------------------------------
+// Global allocator
+// ---------------------------------------------------------------------------
+
+#[global_allocator]
+static ALLOCATOR: preempt::PreemptingAllocator = preempt::PreemptingAllocator;
+
+// ---------------------------------------------------------------------------
+// Public API re-exports
+// ---------------------------------------------------------------------------
+
+pub use channel::{channel, Receiver, RecvError, Sender};
+pub use mutex::{LockTimeout, Mutex, MutexGuard};
+pub use pid::Pid;
+pub use runtime::{init, Config, Runtime};
+pub use scheduler::{
+    block_on_io, run, self_pid, sleep, spawn, spawn_under, wait_readable, wait_writable,
+    yield_now, JoinError, JoinHandle,
+};
+pub use supervisor::Signal;
+
+// ---------------------------------------------------------------------------
+// check!()
+// ---------------------------------------------------------------------------
+
+/// Voluntarily check whether this actor's timeslice has expired, yielding
+/// if so.
+#[macro_export]
+macro_rules! check {
+    () => {
+        $crate::preempt::maybe_preempt()
+    };
+}