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/card_animation/interaction.rs

@@ -72,9 +72,7 @@ pub struct HoverState {
 /// Describes a user action that arrived while cards were still animating.
 #[derive(Debug, Clone)]
 pub enum BufferedInput {
-    Move {
-        from: MoveRequestEvent,
-    },
+    Move { from: MoveRequestEvent },
     Draw,
     Undo,
 }