Make uv sync reproducible on Python 3.12 and macOS

[charliechenye]

What changed

• pin the contributor interpreter to Python 3.12 with .python-version
• move pytest and Ruff from an optional extra into the standard dev dependency group so plain uv sync installs the full development toolchain
• commit a universal uv.lock
• migrate CI from ad-hoc pip installation to uv sync --locked and uv run
• add a macOS ARM64 / Python 3.12 validation job
• update contributor setup instructions for Homebrew and uv

Why

The previous configuration declared development tools under [project.optional-dependencies]. uv does not install project extras during a plain uv sync, which left fresh environments without pytest and Ruff. The new dependency-group configuration makes a clean clone reproducible with one command.

Verification

• uv lock --check
• clean locked sync on a fresh cache
• uv run python -m pytest --version
• uv run ruff --version
• GitHub CI now validates the complete test suite on Ubuntu and macOS ARM64 using Python 3.12