test(engine): add boundary tests for format_secs and pause toggle cycle

leaderboard_plugin: format_secs was missing the 60-second crossing
boundary (1:00 vs 60s), the zero case (0s), and leading-zero padding
verification (1:05 not 1:5).

pause_plugin: added third-press test confirming the Esc toggle is
symmetric across multiple pause/resume cycles.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/pause_plugin.rs

@@ -151,4 +151,28 @@ mod tests {
             0
         );
     }
+
+    #[test]
+    fn toggle_is_symmetric_for_multiple_cycles() {
+        let mut app = headless_app();
+        // Third press re-pauses after resume.
+        press_esc(&mut app);
+        app.update();
+        press_esc(&mut app);
+        app.update();
+        press_esc(&mut app);
+        app.update();
+        assert!(
+            app.world().resource::<PausedResource>().0,
+            "third Esc must re-pause"
+        );
+        assert_eq!(
+            app.world_mut()
+                .query::<&PauseScreen>()
+                .iter(app.world())
+                .count(),
+            1,
+            "third Esc must re-spawn PauseScreen"
+        );
+    }
 }