Add the GitHub-hosted ubuntu-24.04-arm runner to the test matrix so the suite runs natively on arm64 Linux (macos-latest already covers arm64 macOS; build.yml already cross-builds armv7/aarch64 Linux binaries), and document ARM support in docs/arm-support.md.
Fixes#683.
Co-authored-by: Autumn Skerritt <[email protected]>
