feat: Thread cargo build script directives

Summary

• Thread captured cargo build-script directives into downstream granular Rust rustc actions.
• Represent cargo_build_script nodes as first-class graph deps with real action digests and restored directive output paths.
• Move checked-in examples under grouped paths like examples/rust/granular/basic-app and examples/apple/ios/simulator-app, with source code and README files in each project.
• Add examples/rust/granular/build-script-cfg for the build-script scenario, then update shellspec coverage to copy that example into each test workspace and verify cfg propagation, cache hits, and invalidation.

Why

The lower-level TOML graph already allowed [[cargo.build_script]], but dependents did not consume the captured directives. That left build-script-backed crates on the cargo.binary escape hatch even when the graph was declared explicitly.

The build-script scenario is now visible as a normal example project instead of being generated inline inside the shellspec.

Validation

• mise exec -- cargo test --workspace
• mise exec -- cargo fmt --all -- --check
• mise exec -- cargo clippy --workspace --all-targets -- -D warnings
• mise exec -- cargo build --release
• mise exec -- shellspec

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