重构：RustCrypto 兼容接口

主要变更：
- 新增 sm2/、sm3/、sm4/ 独立 crate，实现 RustCrypto traits
  - sm2：实现 signature、elliptic-curve traits
  - sm3：实现 digest、crypto-common traits
  - sm4：实现 cipher、aead traits
- 新增 fuzz/ 模糊测试目标
- 新增 tests/sm2_proptest.rs 属性测试
- 重构 src/sm4/ 使用 RustCrypto AEAD traits
- 更新 CI 工作流支持多 crate 测试
- 更新 .gitignore 忽略 fuzz/target/

sm4/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "sm4"
+version = "0.1.0"
+edition = "2021"
+rust-version = "1.83.0"
+license = "Apache-2.0"
+description = "SM4 (ShangMi 4) block cipher with constant-time bitslice S-box — GB/T 32907-2016"
+repository = "https://github.com/kintaiW/libsmx"
+categories = ["cryptography", "no-std::no-alloc"]
+keywords = ["crypto", "cipher", "sm4", "shangmi", "block-cipher"]
+
+[dependencies]
+cipher  = { workspace = true }
+zeroize = { workspace = true }
+
+[dev-dependencies]
+hex-literal = { workspace = true }
+cipher      = { workspace = true, features = ["dev"] }
+
+[features]
+default = []
+zeroize = []