初始提交：SM2/SM3/SM4/SM9 密码算法库

- SM3 哈希函数 (GB/T 32905-2013)
- SM4 分组密码，支持 ECB/CBC/OFB/CFB/CTR/GCM/CCM/XTS 模式 (GB/T 32907-2016)
- SM2 椭圆曲线密码 (GB/T 32918.1-5-2016)
- SM9 标识密码 (GB/T 38635.1-2-2020)
- 全程常量时间运算
- 支持 no_std，带 alloc 特性
- 完整的国标测试向量
- Criterion 性能基准测试

src/lib.rs

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+//! # libsmx
+//!
+//! Production-grade implementation of Chinese commercial cryptography standards:
+//!
+//! - **SM2** — Elliptic Curve Public Key Cryptography (GB/T 32918.1-5)
+//! - **SM3** — Cryptographic Hash Algorithm (GB/T 32905)
+//! - **SM4** — Block Cipher Algorithm (GB/T 32907)
+//! - **SM9** — Identity-Based Cryptographic Algorithm (GB/T 38635.1-2)
+//!
+//! ## Features
+//!
+//! - `no_std` compatible (requires `alloc` feature for SM2/SM9 operations)
+//! - Constant-time operations via [`subtle`](https://docs.rs/subtle)
+//! - Automatic key zeroization via [`zeroize`](https://docs.rs/zeroize)
+//! - All implementations validated against official GB/T test vectors
+//!
+//! ## Quick Start
+//!
+//! ```rust
+//! use libsmx::sm3::Sm3Hasher;
+//!
+//! let mut h = Sm3Hasher::new();
+//! h.update(b"hello world");
+//! let digest = h.finalize();
+//! assert_eq!(digest.len(), 32);
+//! ```
+//!
+//! ## Security Notice
+//!
+//! This library uses constant-time operations throughout to prevent timing
+//! side-channel attacks. Private keys are zeroized on drop. However, this
+//! library has **not** been independently audited. Use in production at your
+//! own risk.
+//!
+//! ## Standards Compliance
+//!
+//! | Algorithm | Standard |
+//! |-----------|----------|
+//! | SM2 | GB/T 32918.1-5-2016 |
+//! | SM3 | GB/T 32905-2016 |
+//! | SM4 | GB/T 32907-2016 |
+//! | SM9 | GB/T 38635.1-2-2020 |
+
+#![no_std]
+#![forbid(unsafe_code)]
+#![warn(missing_docs, rust_2018_idioms)]
+
+#[cfg(feature = "alloc")]
+extern crate alloc;
+
+pub mod error;
+pub mod sm2;
+pub mod sm3;
+pub mod sm4;
+pub mod sm9;