Python API / PyO3 bindings for embedding Pyrefly

### Summary

Would the project be interested in exposing a Python API for Pyrefly, similar to `mypy.api`?

Currently, the Python package is a CLI wrapper invoking pyrefly via `subprocess`. I'm building a python transpiler and wanted to use pyrefly in CI to verify the generated python code is valid. I had performance concerns calling subprocess as much as I wanted to; wondering if direct bindings exposed to the python api would be better? My guess is yes...

I pictured something like this but it's actually just an FFI to the rust code:

```python
import pyrefly

result = pyrefly.check(paths=["src"])
```

Before spending time on a prototype, I wanted to check whether maintainers would be open to this direction or had other suggestions. I'd love the chance to work on something like this :)

Start by reviewing the Python package's CLI wrapper and how it invokes Pyrefly through subprocess, then inspect the Rust code relevant to a possible PyO3 FFI. Clarify the maintainers' preferred direction and define whether completion means supporting a direct API such as pyrefly.check(paths=["src"]) for CI use.

Written by the indexing model from the issue text.