fix: run the PyO3 runtime through uv

[pesap]

Summary

• Launch the PyO3 payload through a UV-managed virtual environment.
• Use native UV commands to select build Python and create the plugin environment.
• Package the launcher and payload together, and remove custom Python discovery, dylib rewriting, and installer patching scripts.

Acceptance criteria

• UV creates and manages the controlled Python environment.
• R2X project Python runs in the UV-managed environment.
• The installed CLI does not require python3.12 in ~/.local/bin.
• The build and runtime use native UV behavior instead of custom provisioning logic.

Deviations from the original plan

None.

Testing Strategy

• R2X_PYTHON_VERSION=3.12 just test
• R2X_PYTHON_VERSION=3.13 just test
• R2X_PYTHON_VERSION=3.12 just clippy-strict
• bash scripts/ci_check_shell_scripts.sh
• PYTHONPATH=scripts uv run --no-config --no-project --managed-python --python 3.12 -- python -m unittest scripts.tests.test_format_benchmark_summary scripts.tests.test_compare_benchmark_summary
• actionlint .github/workflows/build.yml .github/workflows/release-plz.yml

References

• Original issues: None.
• Files: crates/r2x-cli/src/bin/r2x.rs, crates/r2x-config/src/lib.rs, .github/actions/setup-uv-python/action.yml