Adds the twente-cavitation figures to the Hub, matching the assets folder on GitHub (open-h/OpenH-RF). The card and pipeline are already in sync.

`assets/cavitation_bubbles_10kPa_01mL_per_min.png` is an example Capon (minimum-variance) reconstruction of the matching acquisition, of the kind the Data Validation section describes; `assets/main.png` is the release image. Neither is embedded in the card.

Files changed: 2.

```
+ twente-cavitation/assets/cavitation_bubbles_10kPa_01mL_per_min.png
+ twente-cavitation/assets/main.png
```