Thread cargo build script directives into Rust rustc actions

Build-script-backed crates declared in the lower-level TOML graph now propagate their directives into downstream granular Rust rustc actions, so they no longer force the cargo.binary escape hatch. cargo_build_script nodes are represented as first-class graph dependencies with real action digests and restored directive output paths. A new example project at examples/rust/granular/build-script-cfg shows the build-script scenario in place.