refactor: 优化 NBPF 模块 - 缓存导入/合并重复方法/减少I/O

- crypto.py: 8个_imp_*方法改为_ModuleCache类缓存导入
- crypto.py: outer/inner加解密合并为_layer_encrypt/decrypt
- crypto.py: 提取公共摘要计算方法，拆分长方法
- compiler.py: 删除_obfuscate_code中未使用的死代码
- loader.py: 3次ZIP扫描合并为1次缓存读取
- format.py: 更新为使用_ModuleCache
- 合计减少205行代码（1707→1502）

oss/core/nbpf/format.py

@@ -32,7 +32,7 @@ from pathlib import Path
 from typing import Optional
 
 from oss.logger.logger import Log
-from .crypto import NBPCrypto, NBPCryptoError
+from .crypto import NBPCrypto, NBPCryptoError, _ModuleCache
 from .compiler import NIRCompiler, NIRCompileError
 
 
@@ -148,11 +148,10 @@ class NBPFPacker:
                     zf.writestr(NBPFFormatter.SIGNER_PEM, ed25519_public_key)
                 else:
                     # 从私钥派生公钥
-                    ed25519_mod = NBPCrypto._imp_ed25519()
-                    key = ed25519_mod.Ed25519PrivateKey.from_private_bytes(ed25519_private_key)
+                    key = _ModuleCache.ed25519().Ed25519PrivateKey.from_private_bytes(ed25519_private_key)
+                    s = _ModuleCache.serialization()
                     pub_bytes = key.public_key().public_bytes(
-                        NBPCrypto._imp_serialization().Encoding.Raw,
-                        NBPCrypto._imp_serialization().PublicFormat.Raw
+                        s.Encoding.Raw, s.PublicFormat.Raw
                     )
                     zf.writestr(NBPFFormatter.SIGNER_PEM, pub_bytes)