Deployment: Coverage for both rust and python

[DennisLent]

• Split uploads by language with flags and let Codecov merge
• Rust: keep tarpaulin job uploading Cobertura with flag rust.
• Python: add a pytest-cov job that uploads coverage.xml with flag python. pytest crates/py/tests --cov=crates/py/python/rusty_runways --cov-report=xml:coverage/python.xml --cov-branch
• Add codecov.yml to manage flags and statuses:
• Keep the existing overall badge or per-flag badges