GitHub Actions on push/PR (macOS runner): checks out the vendored SDK
submodule (full history so its pinned fork-branch commit is reachable),
builds the frontend (tauri-build needs ui/dist), then runs
`cargo fmt --check`, clippy (advisory for now), and
`cargo test --workspace`. Caches cargo to keep runs reasonable.
Also point .gitmodules at `tutabridge-integration` (the branch the
submodule commit actually lives on).
