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

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "smarm"
-version = "0.1.0"
+version = "0.3.0"
 edition = "2021"
 rust-version = "1.95"
 
@@ -8,6 +8,7 @@ rust-version = "1.95"
 libc = "0.2"
 
 [dev-dependencies]
+libc = "0.2"
 tokio = { version = "1", features = ["rt", "macros", "sync"] }
 
 [profile.dev]