chore: swap build backend from setuptools to maturin

[SkyeAv]

Swaps the build backend from setuptools to maturin so Babel builds as a mixed Rust/Python
package, ahead of landing native (Rust) functionality. This is the backend swap only — the Rust
side is an inert placeholder module, the importable package stays src, and nothing under it
moves or renames.

Build backend

• pyproject.toml: [build-system] is now requires = ["maturin>=1.0,<2.0"] /
  build-backend = "maturin"; dropped [tool.setuptools.packages.find].
• [tool.maturin]: module-name = "src.rs" places the compiled extension at src/rs.* and
  tells maturin to bundle the src package (auto-resolved to the repo root — src-layout detection
  doesn't fire because there's no src/babel_pipeline/__init__.py); manifest-path = "rust/Cargo.toml" keeps Cargo off the Python src/ dir.
• dev group: added maturin>=1.0,<2.0 so uv run maturin develop works locally (the
  [build-system].requires entry is build-isolation only).

Rust stub

• rust/Cargo.toml + rust/src/lib.rs: empty #[pymodule] fn rs (pyo3, abi3-py311 +
  extension-module), importable as from src import rs. Real functionality lands in a follow-up.
• Layout: Rust lives under rust/ so Cargo's default src/lib.rs never collides with the
  Python src/ package.

Build prerequisites (CI + Docker)

• .github/workflows/test.yml: added dtolnay/rust-toolchain@stable before uv sync in both
  jobs — every uv sync now compiles via cargo, and ubuntu-latest ships no Rust.
• Dockerfile: apt-get install -y cargo (system-wide, before USER nru) for the same reason.
• Accepted caveat: a Rust toolchain is now a build prerequisite for everyone (local, CI,
  downstream installs) — the cost of adopting maturin before the Rust code exists.

Housekeeping

• .gitignore: ignore rust/target/ and the maturin develop binaries
  (src/rs.*.so/.pyd/.dylib).
• uv.lock + rust/Cargo.lock: regenerated and committed (CI runs uv sync --frozen).

Design

• Why maturin needs a stub: maturin is fundamentally a Rust build tool — every layout it
  supports requires a Cargo.toml, so a bare backend swap with no Rust won't build. The empty
  #[pymodule] is the smallest thing that makes maturin viable now.
• Why module-name = "src.rs": the import package is literally named src (flat layout).
  Verified against maturin's resolver (project_layout.rs / module_writer/mod.rs): this bundles
  all of src/* and drops the extension at src/rs.<so> without moving or renaming anything.
• Deferred: real Rust functionality; a manylinux/abi3 publishing matrix (the stub builds a
  local cp311-abi3 wheel).

Testing

• uv sync — builds babel-pipeline via maturin → cargo; installs maturin==1.14.1.
• uv run python -c "import src; from src import rs; from src.tools.clique_diff import cli" — ok.
• uv run pytest -m unit -q → 402 passed, 105 deselected.
• uv build → babel_pipeline-1.17-cp311-abi3-linux_x86_64.whl; unzip -l shows src/__init__.py,
  src/node.py, src/tools/..., src/rs.abi3.so, and all 4 console scripts in entry_points.txt.
• uv run babel-clique-diff --help — resolves.

Questions for the reviewer

• Rust toolchain as a prerequisite. Every build now needs cargo (local, CI, Docker). Fine for a
  stub-only PR, or would you rather gate the maturin switch behind the PR that adds real Rust?

[SkyeAv]

This would allow me to expose rust code directly to python. This backend is the base commit I need before I can start refactoring in that direction

[Copilot]

Pull request overview

This PR switches Babel’s packaging/build backend from setuptools to maturin so the project can build as a mixed Python/Rust distribution, while keeping the importable Python package layout unchanged (src remains the top-level package). It adds a minimal Rust/PyO3 stub extension to satisfy maturin’s requirements and updates CI/Docker to include a Rust toolchain for builds.

Changes:

• Swap PEP 517 build backend to maturin and configure it to bundle the existing src Python package while emitting an extension module at src/rs.*.
• Add a placeholder Rust crate (rust/) with an empty PyO3 #[pymodule] importable as from src import rs.
• Update CI and Docker build prerequisites to install Rust, and add ignores/locks for new build artifacts and dependencies.

Reviewed changes

Copilot reviewed 5 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pyproject.toml	Switch build backend to maturin; configure [tool.maturin]; add dev dependency; adjust ruff formatter config.
uv.lock	Record maturin in the dev dependency set and lock resolution.
rust/Cargo.toml	Define the Rust crate and PyO3 dependency for the placeholder extension module.
rust/src/lib.rs	Add the empty #[pymodule] fn rs stub importable from Python.
rust/Cargo.lock	Commit the Rust dependency lockfile for reproducible cargo builds.
.github/workflows/test.yml	Install a Rust toolchain in CI before uv sync so maturin/cargo builds can run.
Dockerfile	Install cargo in the container image so uv sync can build the maturin extension.
.gitignore	Ignore Rust build outputs and local maturin develop-produced extension binaries.

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