Isolate Rust implementation under top-level rust/ directory

[adulau]

Motivation

• Keep all Rust crates, Cargo manifests, and Rust source files in a single top-level directory so Cargo workspaces and tooling do not get mixed with the Python package.

Description

• Add rust/README.md documenting that all Rust sources, Cargo.toml, Cargo.lock, and *.rs files must live under rust/.
• Add tests/test_repository_layout.py which fails if any *.rs, Cargo.toml, or Cargo.lock file is found outside rust/.
• Update CONTRIBUTING.md to document the repository layout and the rust/ boundary.
• Update .gitignore to ignore Cargo build output by adding target/.

Testing

• Ran git diff --check which produced no whitespace or index errors.
• Ran PYTHONPATH=src python -m pytest -m 'not network' -q and the suite passed with 26 passed, 2 skipped.
• Ran python -m ruff check src tests and lint checks completed without failures.

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Codex Task

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-01T08:36:13.195881Z	9fe5534	PR opened
🔒 Security Review	✅ Completed 2026-09-01T08:37:59.490567Z	9fe5534	PR opened

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