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/achievement_plugin.rs

@@ -911,8 +911,7 @@ mod tests {
 
         // Put the active game in Zen mode. evaluate_on_win reads
         // GameStateResource.mode directly to populate last_win_is_zen.
-        app.world_mut().resource_mut::<GameStateResource>().0.mode =
-            GameMode::Zen;
+        app.world_mut().resource_mut::<GameStateResource>().0.mode = GameMode::Zen;
 
         app.world_mut().write_message(GameWonEvent {
             score: 0,