refactor: 新增 common 子包——XML/JSON 共用部分 + 单 import 全量可用

- src/common/（package simapi_serialization.common）：
  Annotations.cj（@SerializerPropertyName/@SerializerIgnore）、
  ReflectionCache.cj（字段元数据 + 继承链缓存）、NamingPolicy.cj（PropertyNamingPolicy）
- 根锚点 public import common.* + json.*：import simapi_serialization.* 即可用全部
- JsonOption.propertyNamingPolicy 引用 common 枚举；getFields 改收命名策略
- 枚举值 PropertyNamingPolicy.None → Keep（None 与内置 Option.None 构造器同名冲突）
- JsonWriter/JsonReader 单类 import common，避免循环依赖
- 测试/独立项目验证：16 用例全绿；serialization_test 单 import 即可运行

src/json/JsonSerializer_test.cj

@@ -13,9 +13,11 @@ package simapi_serialization.json
 import std.collection.*
 import std.unittest.*
 import std.unittest.testmacro.*
-import simapi_serialization.*
+import simapi_serialization.common.SerializerPropertyName
+import simapi_serialization.common.SerializerIgnore
+import simapi_serialization.common.PropertyNamingPolicy
+import simapi_serialization.common.ReflectionCache
 import simapi_serialization.macros.*
-
 enum Role {
     | Admin
     | User
@@ -237,7 +239,7 @@ public class JsonSerializerTests {
     @TestCase
     public func namingPolicies() {
         let optNone = JsonOption()
-        optNone.propertyNamingPolicy = PropertyNamingPolicy.None
+        optNone.propertyNamingPolicy = PropertyNamingPolicy.Keep
         let jsonNone = JsonSerializer.Serialize(makeUser(), optNone)
 
         let optCamel = JsonOption()
@@ -265,7 +267,7 @@ public class JsonSerializerTests {
         @Expect(ReflectionCache.applyNamingPolicy("Name", PropertyNamingPolicy.CamelCase), "name")
         @Expect(ReflectionCache.applyNamingPolicy("_name", PropertyNamingPolicy.CamelCase), "_name")
         // None：原样
-        @Expect(ReflectionCache.applyNamingPolicy("Name", PropertyNamingPolicy.None), "Name")
+        @Expect(ReflectionCache.applyNamingPolicy("Name", PropertyNamingPolicy.Keep), "Name")
     }
 
     /// 枚举序列化/反序列化