Rust graph toolchain support

Once can now model and build Rust projects using the same graph and action system as other supported ecosystems. It adds Cargo-aware graph rules for first-party libraries, binaries, and proc macros, plus resolved crates and Cargo dependencies. Cargo.toml and Cargo.lock become the source of truth, with a root cargo_dependencies target that runs cargo metadata --locked and feeds resolved external packages into cacheable Once actions. First-party Rust targets now depend on that cacheable Cargo dependency target and use CARGO_PKG_NAME or cargo_package to consume Cargo’s direct external dependencies. The separate generated third-party Once manifest and the once query deps resolver surface were removed. This ships with new documentation for Rust graph rules, including the cargo_dependencies,