Rust toolchain support in the build graph

Rust projects are now first-class citizens in the Once build graph. The toolchain understands Cargo.toml and Cargo.lock as the source of truth, adds built-in target kinds for Rust binaries, libraries, crates, and proc macros, and reconciles resolved external packages through a cacheable cargo_dependencies target. First-party Rust targets can declare Cargo-backed dependencies and use CARGO_PKG_NAME or cargo_package to consume external crates, removing the need for a generated third-party manifest or a separate query resolver.