NodeKind.footnote_ref (typed inline footnote reference)

Surfaced during [pprose](https://github.com/jlevy/practical-prose) structural-metrics work after the chopdiff 0.3.1 upgrade. Not blocking — pprose has a regex workaround — but landing this unifies it with the typed inline walk.

**Need:** A typed inline node kind for footnote references (`[^1]`), so footnote-ref counts come from the same typed inline walk as links / images / code-spans rather than a separate regex.

**Proposed API:** Add `footnote_ref` to `NodeKind`, surfaced via:

```python
collect(kinds={footnote_ref}, inline=True)
```

**pprose workaround today:** Keep pprose's footnote-reference regex.

_Tracked in pprose as `pp-aat4`._