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/cmd/forms.rs

@@ -1716,6 +1716,7 @@ mod tests {
 
     const STATE: &[&str] = &[
         "open a design first",
+        "update checks are off",
         "`boundary_conditions` or `loads`",
         "give one of",
         "is not an explicit",

crates/engine/src/cmd/mod.rs

@@ -25,6 +25,7 @@ pub(crate) mod seeds;
 pub(crate) mod solve;
 mod symmetry;
 mod tags;
+pub mod update;
 pub(crate) mod view;
 
 use crate::schema::ProblemKind;
@@ -158,6 +159,7 @@ pub static ALL: &[CommandSpec] = &[
     diagnostics::CRASH_REPORT,
     diagnostics::BUG_REPORT,
     diagnostics::CRASH_TEST,
+    update::CHECK,
 ];
 
 /// The problem with required parameter `key`: [`ProblemKind::Required`] when it is left out,

crates/engine/src/cmd/update.rs

@@ -0,0 +1,59 @@
+//! `app.update.check`: is a newer FieldCraft published? The same check as the desktop app's
+//! Help › Check for Updates and `fieldcraft-cli update --check` (docs/updates.md). It only
+//! reports: nothing is downloaded or installed, and no dialog opens.
+//!
+//! Checking reaches the network, so it is switched on only by the programs that may
+//! ([`enable_checks`]: the desktop app and the CLI); everywhere else (tests, scratch sessions,
+//! the browser) the command refuses without asking anything.
+
+use std::sync::atomic::{AtomicBool, Ordering};
+
+use serde_json::Value;
+
+use crate::{CommandKind, CommandSpec, EngineError, Result, Session};
+
+pub const CHECK: CommandSpec = CommandSpec {
+    id: "app.update.check",
+    label: "Check for Updates",
+    params: r#"{} — asks GitHub Releases for the latest published FieldCraft release and compares it with this version: current, latest, newer, url (its page and release notes), published, signed (whether it can be verified and installed from the app) and asset (the file this install would download; null when it cannot update itself). Reports only: nothing is downloaded or installed. Desktop app and fieldcraft-cli only"#,
+    form: Some(&[]),
+    kind: CommandKind::Session,
+    requires: &[],
+    run: check,
+};
+
+static ENABLED: AtomicBool = AtomicBool::new(false);
+
+/// Let `app.update.check` reach the release server (the desktop app and the CLI do).
+pub fn enable_checks(on: bool) {
+    ENABLED.store(on, Ordering::SeqCst);
+}
+
+#[cfg(target_arch = "wasm32")]
+fn check(_: &mut Session, _: &Value) -> Result<Value> {
+    Err(EngineError::Params("not in the browser: the web app offers Reload to update when a new version is deployed".into()))
+}
+
+#[cfg(not(target_arch = "wasm32"))]
+fn check(_: &mut Session, _: &Value) -> Result<Value> {
+    if !ENABLED.load(Ordering::SeqCst) {
+        return Err(EngineError::Params("update checks are off in this session (the desktop app and fieldcraft-cli check for updates)".into()));
+    }
+    let check = fieldcraft_update::net::Client::from_env().check().map_err(|e| EngineError::Params(e.to_string()))?;
+    let asset = fieldcraft_update::install::here().asset_name(&check.latest.version, std::env::consts::ARCH);
+    Ok(check.to_json(asset.as_deref()))
+}
+
+#[cfg(test)]
+mod tests {
+    use serde_json::json;
+
+    use super::*;
+
+    /// Off unless a program switches it on: tests and scratch sessions never reach the network.
+    #[test]
+    fn checks_are_off_unless_enabled() {
+        let e = Session::default().execute("app.update.check", &json!({})).unwrap_err();
+        assert!(e.to_string().contains("update checks are off"), "{e}");
+    }
+}