fix(engine): bounded blocking sync push on exit

The exit push spawned a detached task on AppExit, which process
teardown almost always killed before the network round-trip completed —
the final session's sync silently never happened (2026-07-06 review,
finding M3). Local persistence meant no data loss, but stats stayed
unsynced until the next launch.

push_on_exit now blocks the closing app's final frame for at most
EXIT_PUSH_TIMEOUT (2s) via tokio::time::timeout: long enough for one
healthy round-trip, short enough that quitting never feels hung when
the server is unreachable. Timeout and errors are logged and skipped —
the next launch's pull/push converges as before.

Closes review finding M3.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/sync_plugin.rs

@@ -272,12 +272,21 @@ fn poll_pull_result(
     }
 }
 
-/// Last-schedule system: starts a best-effort push of the current local state
-/// on [`AppExit`] without blocking the Bevy main thread.
+/// Upper bound on how long [`push_on_exit`] may block the closing app.
+/// Long enough for one healthy round-trip; short enough that quitting
+/// never feels hung when the server is unreachable.
+const EXIT_PUSH_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(2);
+
+/// Last-schedule system: pushes the current local state on [`AppExit`],
+/// blocking the main thread for at most [`EXIT_PUSH_TIMEOUT`].
 ///
-/// The detached task may be cut short by process teardown, so local atomic
-/// persistence remains the durable source of truth even if the final remote
-/// push does not complete.
+/// This deliberately blocks: the previous detached-task version was almost
+/// always cut short by process teardown, so the final session's push
+/// silently never happened (2026-07-06 review, finding M3). A bounded wait
+/// during the app's final frame is invisible to the player and lets the
+/// round-trip actually complete on a healthy network. On timeout or error
+/// the push is skipped — local atomic persistence remains the durable
+/// source of truth and the next launch's pull/push converges.
 fn push_on_exit(
     mut exit_events: MessageReader<AppExit>,
     provider: Res<SyncProviderResource>,
@@ -292,16 +301,17 @@ fn push_on_exit(
     exit_events.clear();
 
     let payload = build_payload(&stats.0, &achievements.0, &progress.0);
-    let provider = provider.0.clone();
-    let rt = rt.0.clone();
-    AsyncComputeTaskPool::get()
-        .spawn(async move {
-            match rt.block_on(provider.push(&payload)) {
-                Ok(_) | Err(SyncError::UnsupportedPlatform) => {}
-                Err(e) => warn!("sync push on exit failed: {e}"),
-            }
-        })
-        .detach();
+    let result = rt
+        .0
+        .block_on(async { tokio::time::timeout(EXIT_PUSH_TIMEOUT, provider.0.push(&payload)).await });
+    match result {
+        Ok(Ok(_)) | Ok(Err(SyncError::UnsupportedPlatform)) => {}
+        Ok(Err(e)) => warn!("sync push on exit failed: {e}"),
+        Err(_) => warn!(
+            "sync push on exit timed out after {}s; will sync on next launch",
+            EXIT_PUSH_TIMEOUT.as_secs()
+        ),
+    }
 }
 
 /// Update-schedule system: on each `GameWonEvent` push the just-completed