libfw-client: reusable client-side transfer logic and browser glue.

The crate is intentionally split into a reusable Rust library core and a thin wasm bridge. This allows the client logic to be consumed by Rust code directly, while the JS/browser-facing API remains in the dedicated WASM module.

Two client forms, one protocol

• Native Rust ([native::NativeClient], any non-wasm32 target): an async tokio + reqwest transport implementing the same wire protocol — resumable Range downloads, the session upload protocol, zrip compression and the same adaptive tuning engine. Enable it by depending on this crate from a normal Rust binary/library.
• Browser/WASM ([LibfwClient], wasm32 only): the engine behind the sdk/ npm package, which additionally supports the File System Access API and IndexedDB resume state.