Add rust_mobile_library target for sharing Rust code across Apple and Android

A new rust_mobile_library target kind is available in the Rust Starlark prelude. A single Rust target can now be consumed by both Apple and Android app targets in the same project graph. Each consumer only materializes the native library variant it needs, staticlib for Apple and cdylib for Android, so Android-only builds no longer require Apple Rust targets and Apple-only builds no longer require the Android linker. The bundled mobile examples now use a single SharedRust target, and the guide and reference pages document the new target kind. Note: rust_mobile_library does not support Rust dependencies yet, so projects that need those can keep using explicit platform-specific rust_library targets.