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.

---

Read src/ClassModules/SQLSelect.cls, focusing on the declarations and Class_Initialize, then trace the UNION append and iteration paths. Done means the declared array name matches its references, it is initialized before UNION use, and the reproduction steps no longer fail or produce incorrect SQL.

Escrito por el modelo de indexación a partir del texto del issue.