SQLSubselect — Property Get returns wrong variable

SQLSubselect: SelectAs getter returns wrong variable
- Component: `src/ClassModules/SQLSubselect.cls` (around 0–39)

### Problem
- Getter returns an undefined identifier instead of the backing field.

### Snippet (actual)
```vb
Property Get SelectAs() As String
SelectAs = aAs
End Property
```

### Expected
```vb
Property Get SelectAs() As String
SelectAs = sAs
End Property
```

### Steps to Reproduce
1) Read `SelectAs` from an instance.

### Actual
- Returns empty/undefined; potential compile/runtime error.

### Proposed Fix
- Return `sAs`.

---

打开 src/ClassModules/SQLSubselect.cls，检查第 0–39 行附近的 SelectAs 属性 getter，并将其与该类使用的后备字段进行比较。验证从实例读取 SelectAs 时会返回存储的别名值，不会得到空结果，也不会出现编译/运行时错误。

由索引模型根据 Issue 内容生成。