FFI-independent helpers shared by the Lindera language bindings
(lindera-python, lindera-php, lindera-ruby, lindera-nodejs,
lindera-wasm).
Each binding wraps the core lindera crate for its own FFI layer (PyO3,
ext-php-rs, magnus, napi, wasm-bindgen) and historically reimplemented the
same pure-Rust conversion logic. This crate holds that shared logic as
plain Rust so it can be unit-tested without any FFI toolchain.