Cache Rust compilation by generated output content

Rust compilation can now reuse cached artifacts after a generator rebuilds without changing its generated files or compiler directives, provided every other compiler input is unchanged.

Build-script output directories also participate in downstream cache keys and input materialization. Changing a generated native library now relinks its consumers even when the Rust sources and build-script directives stay the same. Restoring a cached build includes those generated artifacts.

Build scripts keep conservative package-wide input tracking. Generated files that contain changing timestamps still invalidate their consumers.