Add CI check for generated Python stubs #8
Description
Context
The Python stubs file (bindings/python/formualizer/__init__.pyi) is generated by cargo run --bin stub_gen and can drift from Rust bindings.
Proposal
Add a CI job that regenerates stubs and fails if there is a diff.
Acceptance criteria
- Add a CI step that runs stub generation in
bindings/python - If generated stubs differ, CI fails with a clear message
- Document local command needed to refresh stubs
- Ensure workflow works with current Python runtime setup in CI