SQLSelect — UNION storage/rendering broken

SQLSelect: fix UNION storage/rendering and clause order
- Component: `src/ClassModules/SQLSelect.cls`

### Problems
- Stores the class identifier instead of the instance in `Union`.
- Name mismatch between `sUnion` and `aUnion`; union array not initialized.
- `UnionString` builds only one union; ORDER BY placed before UNION in final SQL.

### Snippet (actual)
```vb
Public Sub Union(oSelect As SQLSelect, Optional sType = "")
Dim UnionArray() As Variant
UnionArray = Array(SQLSelect, sType) ' uses class name, not instance
' ...
End Sub

Private Function UnionString()
Dim NewSelect As iSQLQuery
aUnion = UnionArray(0)
Set NewSelect = aUnion(0)
UnionString = " UNION " & NewSelect.toString()
End Function
```

### Expected (conceptual)
```vb
' Store (oSelect, sType)
aUnion = ArrayPush(aUnion, Array(oSelect, sType))

' Build all unions and place before ORDER BY
Function UnionString() As String
Dim i As Long, s As String
For i = 0 To UBound(aUnion)
Dim it As Variant: it = aUnion(i)
Dim q As iSQLQuery: Set q = it(0)
Dim t As String: t = it(1)
s = s & " UNION " & IIf(t <> "", t & " ", "") & q.toString()
Next i
UnionString = s
End Function
```

### Steps to Reproduce
1) Create two selects; call `First.Union Second` and then `OrderBy`.

### Actual
- UNION omitted/malformed; `ORDER BY` rendered without merging unions.

### Proposed Fix
- Store `(oSelect, sType)`; iterate unions; emit before `OrderByString`; initialize `aUnion`.

---

从 src/ClassModules/SQLSelect.cls 开始，检查 Union、UnionString、aUnion 以及最终的 SQL 渲染顺序。使用两个 SELECT 后跟 OrderBy 重现该问题，然后验证 unions 已初始化、所有 unions 都已渲染，并且 UNION 子句出现在 ORDER BY 之前。

由索引模型根据 Issue 内容生成。