Systemd merges support for programs written in Rust

Systemd has merged optional support for writing programs in Rust, built with meson rather than cargo. Rust code uses only core, with no std, alloc or external crates, and links libsystemd-shared dynamically while the library itself stays C. It needs rustc 1.85, bindgen 0.71.1 and meson 1.4.0, and defaults to auto-detection. A parity test compares a C and a Rust test command's output byte for byte. Cross-compilation is not yet supported, and the stated plan is Rust for new leaf components only.