Lingva CLI engine

The crates.io package is named lingva. This crate is the single implementation of Lingva's customer CLI. It builds the standalone lingva executable used by direct downloads, Homebrew, npm, and PyPI. Language packages may locate and launch this binary, but must not reimplement CLI behavior.

Owned commands:

• lingva auth login|use-key|status|logout
• lingva init
• lingva update
• lingva sync|check
• lingva pull|push

The TypeScript CLI remains available during the compatibility window for operator and artifact-generation commands that are not part of this customer surface. Remove that compatibility layer only after the Rust parity gate covers every command retained in the public contract.