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_plugin/highlights.rs

@@ -2,7 +2,6 @@
 
 use super::*;
 
-
 use bevy::color::Color;
 use solitaire_core::Card;
 use solitaire_core::game_state::GameState;
@@ -272,5 +271,3 @@ pub(super) fn find_top_card_at(
 // ---------------------------------------------------------------------------
 // Task #28 — Stock-empty visual indicator
 // ---------------------------------------------------------------------------
-
-