Migrate build system from setuptools to maturin

[dmartmillan]

Description
Switch pyproject.toml build backend from setuptools to maturin. Verify that maturin develop produces a .so importable as openvariant._core.

Files

• pyproject.toml — MODIFY (build-system, tool.maturin sections)
• MANIFEST.in — review for maturin compat

Definition of Done

• pip install maturin && maturin develop completes without error
• python -c "import openvariant._core; print('ok')" prints ok
• Existing pytest tests/ still passes (Python-only tests must not break before any logic is moved)
• maturin build --release produces a .whl file

Documentation needed: Update README.md — replace pip install -e . with maturin develop --release in the developer setup section.

Tests needed: Add a smoke test tests/test_core_import.py asserting import openvariant._core succeeds.

Depends on: #61