Add a shared rust_mobile_library target for Apple and Android consumers

The Rust Starlark prelude now includes a rust_mobile_library target kind that lets a single shared Rust dependency be consumed by both Apple and Android app targets in the same project graph. The target records its source and crate metadata, and each platform consumer materializes only the native library variant it needs, such as staticlib for Apple and cdylib for Android. This means Android-only builds no longer need the Apple Rust target, and Apple-only builds no longer need the Android linker, while the shared Rust target still appears as a single node in the dependency graph. The change includes updated documentation and examples that replace separate Apple and Android Rust targets with one SharedRust target. Note that rust_mobile_library currently rejects Rust dependencies until per-â