Fude

Scaffold a new opt-in feature for the fude crate following the with_* builder convention — new module in src/, App::with_<feature> method, integration tests in tests/, inline tests only for private helpers. Use when adding a new capability like with_tray, with_updater, etc.

Run the full pre-release gate for the fude crate (fmt, clippy strict, tests, doctests, rustdoc, publish dry-run, CHANGELOG sanity). Use before tagging or publishing to crates.io.