Add embeddable client lifecycle API

[wukeying]

Summary

• add client::run_async_with_shutdown for caller-owned graceful shutdown
• add RusnelHandle with start/stop/state/subscribe for embedded async hosts
• add an end-to-end embeddable client test covering forward and reverse TCP tunnels

Validation

• cargo fmt --all -- --check
• cargo clippy --lib --bins -- -D warnings
• cargo test stop_is_ok_when_idle
• cargo test --test embeddable_client -- --nocapture

Note: full all-target clippy and full cargo test currently hit existing test-suite issues unrelated to this change on Windows.