Stable Rust compilation on Windows through graph actions

Windows users can now reliably compile Rust dependencies through Once’s graph actions, including crates with large dependency graphs such as once-core. The full rustc invocation is routed through a response file on Windows so the OS command line limit is no longer hit, feature cfg values and crate-level arguments are written verbatim, host tool lookup honors PATHEXT candidates while skipping extensionless shell shims, and build-script crates avoid the Git Bash wrapper when invoking the native compiler.