Granular Rust builds now propagate cargo build-script directives

Crates that use build.rs files now work with the granular Rust build graph instead of falling back to the cargo.binary escape hatch. The directives they emit, such as cfg flags and link arguments, are captured and threaded into downstream rustc actions. A new examples/rust/granular/build-script-cfg example demonstrates the scenario, and Shellspec coverage verifies cfg propagation, cache hits, and invalidation.