Mobile Plonky2 crate fails to build

# Description

To reproduce:

```
cd mobile/plonky2
cargo check
```

There is a workspace resolution error.

Additionally, multiple files would still reference the old crate name `plonky2_sha256`, should be updated to `plonky2_circuits` if #238 is merged.

Start in mobile/plonky2 and run cargo check to reproduce the workspace resolution error. Review the references to plonky2_sha256 and the proposed plonky2_circuits name, then check whether #238 has merged; done means the crate builds and the stale references are updated if that rename is in scope.

Written by the indexing model from the issue text.