refactor(core): delegate check_win to Session::is_win

check_win() manually projected through session.state().state().is_win(),
reaching the inner Klondike's is_win. Session::is_win() (card_game 0.4.1)
wraps that exact same projection, so collapse the three-hop reach into a
single-hop delegation. check_auto_complete() keeps its projection because
is_win_trivial() is a Klondike-only method with no Session wrapper.

Behavior is bit-for-bit identical; the test-support override in
is_won()/is_auto_completable() (which call check_win) is untouched.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

solitaire_core/src/game_state.rs

@@ -969,7 +969,7 @@ impl GameState {
 
     /// Returns `true` when all four foundation slots each contain a complete A→K sequence.
     pub fn check_win(&self) -> bool {
-        self.session.state().state().is_win()
+        self.session.is_win()
     }
 
     /// Returns `true` when the game can be completed without further player input