Engine: app.update.check reports whether a newer release is published (#231)

The same check as the app's Help › Check for Updates and fieldcraft-cli update --check: the
latest release, whether it is newer, its page, whether it is signed and the asset this install
would download. It only reports. Checking reaches the network, so only the desktop app and the
CLI switch it on; tests, scratch sessions and the browser build refuse without asking.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/lib.rs

@@ -21,6 +21,7 @@ pub mod logfile;
 
 pub use cmd::analytic_surfaces;
 pub use cmd::replace::{MESHERS, Replacement, replacement};
+pub use cmd::update::enable_checks as enable_update_checks;
 pub use fieldcraft_io::{Item as ViewItem, ItemKind};
 pub mod history;
 pub mod progress;