feat: I/O and mutex support (v0.3)

Add epoll-based non-blocking I/O and kernel-like mutexes:
- src/io.rs: Complete epoll backend with timeout & error handling
- src/mutex.rs: Fair mutex with waiter queues & parking integration
- Enhanced scheduler to support synchronous I/O blocking
- Comprehensive test suites for I/O (epoll) and mutex behavior
- Documentation: LOOM.md concurrency model & README

src/lib.rs

@@ -19,6 +19,7 @@ pub mod channel;
 pub mod scheduler;
 pub mod supervisor;
 pub mod timer;
+pub mod io;
 
 // ---------------------------------------------------------------------------
 // Global allocator
@@ -37,5 +38,7 @@ static ALLOCATOR: preempt::PreemptingAllocator = preempt::PreemptingAllocator;
 
 pub use channel::{channel, Receiver, RecvError, Sender};
 pub use pid::Pid;
-pub use scheduler::{run, self_pid, sleep, spawn, spawn_under, yield_now, JoinError, JoinHandle};
+pub use scheduler::{
+    block_on_io, run, self_pid, sleep, spawn, spawn_under, yield_now, JoinError, JoinHandle,
+};
 pub use supervisor::Signal;