fix(structgen): lookup_field SHA1 隔离临时缓冲改用 stdlib.malloc（lookup_field 无 pool 参数）并补 free

App/lib/core/Gen/StructGenMixin.vp

@@ -722,7 +722,7 @@ class StructGenMixin:
                 pre_len_lf: t.CSizeT = t.CSizeT(
                     t.CUInt64T(dot_ty_lf) - t.CUInt64T(ty_name))
                 if pre_len_lf == 16:
-                    sha_buf_lf: str = pool.alloc(17)
+                    sha_buf_lf: str = stdlib.malloc(17)
                     if sha_buf_lf is not None:
                         string.strncpy(sha_buf_lf, ty_name, 16)
                         sha_buf_lf[16] = '\0'
@@ -746,9 +746,13 @@ class StructGenMixin:
                             fe: GenTypes.FieldEntry | t.CPtr = self._get_field_entry(entry, fi)
                             if fe is not None and fe.Name is not None:
                                 if string.strcmp(fe.Name, field_name) == 0:
+                                    if ty_sha1_lf is not None:
+                                        stdlib.free(ty_sha1_lf)
                                     return fe
                         # 名称匹配但字段未找到，继续检查其他同名 entry（跨模块同名）
                         continue
+        if ty_sha1_lf is not None:
+            stdlib.free(ty_sha1_lf)
         # 所有条目都未匹配
         return None