Support platform-specific crate annotations

[finn-ball]

Summary

• add platform-selectable deps, link_deps, and target_compatible_with to crate annotations
• resolve native link_deps as labels in the module that declares the annotation
• allow exact removal of generated normal and build-script dependencies
• preserve rustc-source repository compatibility and document the new controls
• add focused unit coverage plus generated-repository integration tests for selected and removed dependencies

Why

Cargo metadata sometimes needs Bazel-specific platform corrections or dependency replacement without patching generated repositories. Label-typed native dependencies preserve repository identity when annotations are declared outside the generated crate repository.