Notes on incrementally presented slide are hidden until end

### Describe the bug

I attempted to incrementally introduce some bullet points as shown in the example slide deck.

For that to be usable, I'd like the speaker notes to be visible for all of the displayed increments.

They're currently hidden until the last increment is shown.

### How to reproduce the bug

1. Add a slide with both speaker notes and incremental elemental presentation
2. Step through the slide deck
3. Speaker notes for the incremental presentation slide are hidden until the last increment is displayed

### Minimum example code

```shell

```

### Screenshots

_No response_

### Environment details

- Operating system and version: Fedora 43
- Python version: 3.14
- Software versions:
- Briefcase: 0.4.1
- Toga: 0.5.1
- Podium: main branch (https://github.com/beeware/podium/commit/c9ee3c71f3c547eaede4f09b28ebdcd682fee658)

### Logs

_No response_

### Additional context

_No response_

Start by reproducing the issue with a slide containing speaker notes and incrementally presented bullet points, then trace how the presentation advances and when notes are displayed. Done means the speaker notes remain visible for every displayed increment rather than appearing only at the end.

Written by the indexing model from the issue text.