Visual artifacts when using the Expression Sampling example

Hi!
Great project, and I'm very happy to see GNM released open source!

I was running the [Expression Sampling](https://github.com/google/GNM/blob/main/gnm/shape/README.md#expression-sampling) example from the documentation and noticed some visual artifacts in the generated mesh.

In particular, I can see irregular geometry around the eyes and cheek area on both sides, as shown in the attached screenshots.

I reproduced this using the example code as provided in the README, without modifying the sampled expression parameters.

Is this expected behavior, or could there be an issue with the current model/data or the example?

Thanks!

Environment:
- macOS
- Python 3.13.15
- GNM v3
- PyTorch: 2.14.0

Start with gnm/shape/README.md#expression-sampling and reproduce the reported mesh artifacts using the provided example and environment details. Compare the generated geometry around the eyes and cheeks with the attached screenshots; done means determining whether this is expected or identifying whether the example, model, or data is responsible.

Written by the indexing model from the issue text.