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/

fuzz/Cargo.toml

@@ -0,0 +1,37 @@
+[package]
+name = "libsmx-fuzz"
+version = "0.0.0"
+publish = false
+edition = "2021"
+
+[package.metadata]
+cargo-fuzz = true
+
+# Reason: 空 [workspace] 使 fuzz crate 独立于根 workspace，
+# 这是 cargo-fuzz 的标准做法（需要 nightly 且不应影响 stable CI）。
+[workspace]
+
+[dependencies]
+libfuzzer-sys = "0.4"
+libsmx = { path = ".." }
+sm2 = { path = "../sm2" }
+sm3 = { path = "../sm3" }
+sm4 = { path = "../sm4" }
+
+[[bin]]
+name = "fuzz_sm3_digest"
+path = "fuzz_targets/fuzz_sm3_digest.rs"
+test = false
+doc = false
+
+[[bin]]
+name = "fuzz_sm4_roundtrip"
+path = "fuzz_targets/fuzz_sm4_roundtrip.rs"
+test = false
+doc = false
+
+[[bin]]
+name = "fuzz_sm2_verify"
+path = "fuzz_targets/fuzz_sm2_verify.rs"
+test = false
+doc = false