SQLSelect — union array not initialized & name mismatch

SQLSelect: initialize aUnion and fix name mismatch
- Component: `src/ClassModules/SQLSelect.cls` (declarations and initializer)

### Problem
- Declares `sUnion()` while other code references `aUnion()`; not initialized in `Class_Initialize`.

### Expected
```vb
Private aUnion() As Variant

Private Sub Class_Initialize()
ReDim aJoin(0)
ReDim vFields(0)
ReDim aOrder(0)
ReDim aUnion(0)
' ...
End Sub
```

### Steps to Reproduce
1) Use UNION features without prior array setup.

### Actual
- Append/iteration fails or produces wrong SQL.

### Proposed Fix
- Rename to `aUnion` and `ReDim` in initializer.

---

阅读 src/ClassModules/SQLSelect.cls，重点查看声明和 Class_Initialize，然后跟踪 UNION 的追加和迭代路径。完成的标准是：声明的数组名称与其引用一致，在使用 UNION 之前完成初始化，并且复现步骤不再失败或生成错误的 SQL。

由索引模型根据 Issue 内容生成。