This commit adds a comprehensive CI/CD pipeline that runs on all pull requests
and pushes to main/master branches.

## Workflow Features

### Rust Checks (Multi-OS)
- Runs on Ubuntu and macOS
- Checks Rust code formatting with `cargo fmt --check`
- Runs Clippy linter (warnings displayed but not enforced)
- Runs all Rust unit tests with `cargo test`
- Uses caching for faster builds

### Elixir Tests (Matrix Testing)
- Tests on multiple Elixir versions (1.17.0, 1.18.0)
- Tests on multiple OTP versions (26.2, 27.0)
- Tests on Ubuntu and macOS
- Checks Elixir code formatting with `mix format --check-formatted`
- Compiles with warnings as errors
- Runs full Elixir test suite
- Uses caching for Mix and Cargo dependencies

### Integration Tests
- Runs comprehensive integration tests with `mix test --trace`
- Ensures full project compilation works
- Only runs after Rust and Elixir checks pass

### Final Gate Check
- Ensures all jobs passed before merging
- Provides clear status for PR reviews

## Benefits

1. **Prevents Breaking Changes**: Catches issues before they reach main
2. **Multi-Platform Testing**: Ensures code works on Linux and macOS
3. **Version Compatibility**: Tests against multiple Elixir/OTP versions
4. **Fast Feedback**: Caching makes subsequent runs much faster
5. **Clear Status**: Easy-to-read job summaries in PR checks

## Usage

The workflow runs automatically on every PR. You can also:
- View workflow runs in the "Actions" tab on GitHub
- Re-run failed jobs from the Actions tab
- See detailed logs for each step

This provides a solid foundation for maintaining code quality as the project
grows and accepts contributions.