style: cargo fmt under rustfmt 1.9 and gate formatting in CI

The repo was formatted under an older stable; rustfmt 1.9 (Rust 1.95)
wraps signatures and call sites differently, so every touched file was
picking up unrelated formatting hunks. One mechanical pass, and a
'cargo fmt --check' step in the test workflow (same pinned 1.95.0
toolchain) so drift can't accumulate again.

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

solitaire_engine/src/stats_plugin.rs

@@ -1500,10 +1500,8 @@ mod tests {
     #[test]
     fn zen_win_event_updates_zen_best_score_only() {
         let mut app = headless_app();
-        app.world_mut()
-            .resource_mut::<GameStateResource>()
-            .0
-            .mode = solitaire_core::game_state::GameMode::Zen;
+        app.world_mut().resource_mut::<GameStateResource>().0.mode =
+            solitaire_core::game_state::GameMode::Zen;
 
         app.world_mut().write_message(GameWonEvent {
             score: 1800,