ci: add GitHub Actions workflow and Codecov integration

[mattwend]

Summary

Add a CI pipeline and code coverage reporting for the project.

Changes

GitHub Actions workflow (.github/workflows/ci.yml)

• Formatting — runs cargo fmt -- --check to enforce consistent style
• Clippy — runs cargo clippy with -D warnings across all targets and features
• Tests — runs cargo test --all-features
• Coverage — generates an LCOV report via cargo-llvm-cov and uploads it to Codecov

All jobs use dtolnay/rust-toolchain@stable and Swatinem/rust-cache@v2 where applicable.

The workflow triggers on pushes to main and on all pull requests.

Codecov configuration (codecov.yml)

• Configures project and patch coverage targets
• Disables fail_ci_if_error so coverage upload issues don't block the pipeline