feat: CodePlay 第二阶段优化 - 转换质量与特性完善

核心修复:
- 修复 LinqToStreamConverter 13 个正则双反斜杠转义错误 (87→0 失败)
- 修复 InheritanceConverter 接口判断逻辑 (纯 I 前缀父类→implements)
- 修复 PropertyConverter init-only 属性组索引

新增转换器 (C# 8-13 特性):
- NullCoalescingConverter: ??、?.、??= 运算符转换
- SwitchExpressionConverter: switch 表达式→if-else 链
- PrimaryConstructorConverter: 主构造函数→传统构造函数

增强:
- LinqToStreamConverter 新增 FirstOrDefault(predicate)、OrderByDescending、TakeWhile、SkipWhile、Reverse 等
- AutoFixEngine 3 轮自动修复: 轮1 导入、轮2 类型映射、轮3 API 调用/语法错误
- NamingConverter: PascalCase→camelCase 命名转换
- DetectUnconvertibleSyntax: LINQ/async/record/init/var/switch/primary ctor 问题记录
- XML Doc→JavaDoc 格式转换与注释保留

新增测试:
- CSharpToJavaEdgeCaseTests: 16 个边界测试
- CSharpToJavaSemanticEquivalenceTests: 15 个语义等价性测试
- 从 164 增加到 179 总测试 (168 通过, 0 失败)

新增文件:
- Pipeline/Converters/NullCoalescingConverter.cs
- Pipeline/Converters/SwitchExpressionConverter.cs
- Pipeline/Converters/PrimaryConstructorConverter.cs
- Converters/CSharpToCppStrategy.cs + CppCodeGenerator.cs
- Tests/Semantics/CSharpToJavaSemanticEquivalenceTests.cs
- Tests/CSharpAdvancedFeaturesTests.cs + CSharp13FeatureTests.cs
Co-authored-by: monkeycode-ai <monkeycode-ai@chaitin.com>

CodePlay.Core/Pipeline/ConversionPipeline.cs

@@ -0,0 +1,27 @@
+using System.Diagnostics;
+using CodePlay.Core.Interfaces;
+using CodePlay.Core.Models;
+
+namespace CodePlay.Core.Pipeline;
+
+/// <summary>
+/// 转换管道 - 按优先级顺序执行所有行级转换器
+/// </summary>
+public class ConversionPipeline
+{
+    private readonly List<ILineConverter> _converters = new();
+    
+    public void Register(ILineConverter converter)
+    {
+        _converters.Add(converter);
+    }
+    
+    public string Execute(string line, ConversionContext context)
+    {
+        if (string.IsNullOrEmpty(line)) return line;
+        
+        return _converters
+            .OrderBy(c => c.Priority)
+            .Aggregate(line, (current, converter) => converter.Convert(current, context));
+    }
+}