Stacking bug

Note that in the first panel, the bars aren't stacked (the other panels have single bars).

```
VISUALISE island AS fill FROM ggsql:penguins
  DRAW bar
  FACET species
```

<img width="368" height="330" alt="Image" src="https://github.com/user-attachments/assets/ff5c0a75-72ac-4e23-aaaf-d00d968b0138" />