feat(engine,core): add elapsed-time tick system and Zen GameMode

Phase 6 part 4 (partial):
- GameState now tracks elapsed_seconds via tick_elapsed_time in GamePlugin
  (per-second increment while not won). Pure helper advance_elapsed makes
  the tick logic directly testable without mocking Bevy Time.
- New GameMode enum (Classic / Zen) on GameState. Zen mode suppresses
  scoring in move_cards and undo. GameState::new_with_mode allows callers
  to construct non-Classic games; the existing GameState::new still
  defaults to Classic. mode is serde(default) for backwards-compatible
  persistence.
- NewGameRequestEvent gains an optional mode field; handle_new_game
  honours it (falling back to the current game's mode when None).
- InputPlugin: pressing Z starts a fresh Zen-mode game.

Time Attack, Challenge mode, level-5 unlock gating, and unlock UI are
still deferred.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/src/input_plugin.rs

@@ -69,7 +69,13 @@ fn handle_keyboard(
         undo.send(UndoRequestEvent);
     }
     if keys.just_pressed(KeyCode::KeyN) {
-        new_game.send(NewGameRequestEvent { seed: None });
+        new_game.send(NewGameRequestEvent::default());
+    }
+    if keys.just_pressed(KeyCode::KeyZ) {
+        new_game.send(NewGameRequestEvent {
+            seed: None,
+            mode: Some(solitaire_core::game_state::GameMode::Zen),
+        });
     }
     if keys.just_pressed(KeyCode::KeyD) {
         draw.send(DrawRequestEvent);