Support cargo build scripts in granular Rust graphs

Granular Rust builds can now consume cargo build script directives declared in the lower-level TOML graph. When a [[cargo.build_script]] is declared, its captured directives (cfg flags, env, linker args, etc.) are threaded into the downstream rustc actions for dependents, so build-script-backed crates no longer have to fall back to the cargo.binary escape hatch. Build scripts are also represented as first-class graph dependencies with real action digests and restored directive output paths, propagating correct cache invalidation across the graph.