Update pyproject.toml for maturin + finalize metadata

[dmartmillan]

Description
Produce the final pyproject.toml with maturin build backend, updated [project] metadata (version 2.0.0), abi3-py38 wheel configuration, and [project.optional-dependencies] for dev extras.

Files

• pyproject.toml — FINAL UPDATE

Definition of Done

• maturin build --release produces a single abi3 wheel for Linux, macOS, Windows
• pip install open_variant-2.0.0-*.whl works in a clean virtualenv
• openvar --version prints 2.0.0
• strip = true reduces wheel size

Documentation needed: Update installation section in README.md — note that wheels are now pre-compiled; source installs require Rust toolchain.

Tests needed: CI matrix: install wheel in Python 3.8, 3.10, 3.12 and run smoke tests.