Rust clippy lints

Runs `cargo clippy` over Rust crates and workspaces. NOT safe on untrusted code: clippy compiles the crate, so it runs build.rs scripts, proc-macros and any .cargo/config.toml hooks of the target, which is why it is opt-in. Runs with --locked --offline (needs a committed Cargo.lock and already vendored or cached dependencies; nothing is downloaded) and a throwaway target dir. clippy::correctness findings are high, suspicious medium, perf and complexity low, style info; compile errors are reported with low confidence.