Set up GitHub Actions CI for maturin wheel builds

[dmartmillan]

Description
Add .github/workflows/ci.yml to build and test wheels across the full matrix, and .github/workflows/release.yml to publish to PyPI on tag.

Files

• .github/workflows/ci.yml — NEW/UPDATE
• .github/workflows/release.yml — NEW
  CI matrix
• OS: ubuntu-latest, macos-latest, windows-latest
• Python: 3.8, 3.10, 3.12
• Jobs: cargo test, maturin build --release, pytest tests/

Definition of Done

• All matrix combinations green
• Benchmark regression gate wired in (see build(deps): bump requests from 2.28.1 to 2.32.2 #34)
• maturin publish triggered on v* tags

Documentation needed: Update CONTRIBUTING.md — explain CI matrix and how to reproduce locally.

Tests needed: CI itself is the test; verify all matrix jobs pass.