## MinimalTimerExporter 架构重构：类封装方案

### 目标
将散落在 namespace 中的结构体和函数封装为类，提升代码可读性、可测试性和可维护性。

---

### 变更清单

#### 1. FExportContext 移入头文件

```cpp
struct FExportContext
{
    const Trace::IAnalysisSession& Session;
    const Trace::ITimingProfilerProvider* Provider = nullptr;
    const Trace::ITimingProfilerTimerReader* TimerReader = nullptr;
    FExportFilter Filter;
    TMap<uint32, uint32> TimelineIndexToThreadId;

    FExportContext(const Trace::IAnalysisSession& InSession);
    void Initialize(const Trace::ITimingProfilerProvider* InProvider, const FExportFilter& InFilter);
    uint32 GetThreadId(uint32 TimelineIndex) const;
};
```

#### 2. DecodeMetadataToString → FTraceExportUtils

```cpp
class FTraceExportUtils
{
public:
    static void DecodeMetadataToString(FString& Str, const TArrayView<const uint8>& Metadata);
};
```

#### 3. 创建 FTraceExportProcessor 类

```cpp
class FTraceExportProcessor
{
public:
    FTraceExportProcessor(FExportContext& InContext, IEventWriter& InWriter, ChunkingFileWriter& InChunkWriter);

    void Run();

private:
    void ProcessSingleTimeline(uint32 TimelineIndex);
    void ProcessSingleEvent(...);
    void CheckAndRotateChunk();

    FExportContext& Context;
    IEventWriter& Writer;
    ChunkingFileWriter& ChunkWriter;
    uint32 EventsPerChunk = 0;
};
```

#### 4. 简化 FMinimalTimerExporter

```cpp
class FMinimalTimerExporter
{
public:
    static bool ExportTimingEventsToCSV(...);
    static bool ExportTimingEventsToJSON(...);
    static void StreamEventsWithTreeBuilding(...);

private:
    static void SetupExport(...);
};
```

---

### 预期效果

| 指标 | 改进前 | 改进后 |
|-----|-------|-------|
| RunExport 参数 | 3个（Context, Writer, ChunkWriter） | 0个（成员变量） |
| 代码复用 | 各导出函数重复遍历逻辑 | 统一由 Processor 处理 |
| 可测试性 | 难以单元测试 | Processor 可独立测试 |

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是否按此方案进行重构？