refactor(core): derive score/undo/recycle from upstream session stats

Replace the bespoke WXP scoring engine with the upstream
card_game/klondike session stats, eliminating duplicated state that
could drift from the single source of truth.

score()/undo_count()/recycle_count() now read session.stats(); the -15
undo penalty is configured as SessionConfig::undo_penalty and applied by
the upstream score formula. Save schema bumped v4 -> v5 (the three
counters are no longer persisted -- they are rebuilt by replaying the
forward instruction history on load).

- Remove GameState fields score, undo_count, recycle_count (#87)
- Remove score_history / is_recycle_history undo journal (#86)
- Remove KlondikeAdapter::apply_undo_score and the score_for_* helpers,
  plus pre_instruction_score_delta / will_flip_tableau_source (#84)

These three issues are a single atomic change: each removed field/helper
is consumed by the same draw/apply_instruction/undo/serde/PartialEq
paths, so they cannot compile or pass tests in isolation.

Behaviour changes (intentional): the escalating recycle penalty and
per-step score floor are gone (upstream linear scoring, floored once at
0); recycle_count is now cumulative; undo_count resets across save/load.

Refs #84, #86, #87

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

solitaire_engine/src/game_plugin.rs

@@ -920,7 +920,7 @@ fn handle_move(
                 changed.write(StateChangedEvent);
                 if !was_won && game.0.is_won() {
                     won.write(GameWonEvent {
-                        score: game.0.score,
+                        score: game.0.score(),
                         time_seconds: game.0.elapsed_seconds,
                     });
                     // Delete the saved state — a won game should not be resumed.
@@ -1117,7 +1117,7 @@ fn check_no_moves(
         // Only spawn the overlay if one does not already exist, and no other
         // modal scrim is currently open (global ModalScrim guard).
         if game_over_screens.is_empty() && scrims.is_empty() {
-            spawn_game_over_screen(&mut commands, game.0.score, font_res.as_deref());
+            spawn_game_over_screen(&mut commands, game.0.score(), font_res.as_deref());
         }
     }
 }