Nothing in CI compiled this crate. `.rs` lives under `apps/`, so the
change classifier matched a Rust edit as `frontend` and ran the
TypeScript matrix, which cannot notice a Rust error — the crate's 58 unit
tests had never executed once, and neither would the pipe-drain tests in
the previous commit.
Adds a `rust` lane and a Linux `cargo test --lib` job. Linux on purpose:
the pipe-drain fixtures need a real process tree whose grandchild
inherits the parent's stdout and are `#[cfg(unix)]`, so a Windows runner
would compile them out and report green over zero coverage. The Windows
half of that contract is `-SelfTestPipeDrain` on the existing Windows
lane.
5caea5e501