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/events.rs

@@ -1,6 +1,7 @@
 //! Cross-system events used by the engine's plugins.
 
 use bevy::prelude::Event;
+use solitaire_core::game_state::GameMode;
 use solitaire_core::pile::PileType;
 
 /// Request to move `count` cards from `from` to `to`. Fired by input systems,
@@ -21,9 +22,11 @@ pub struct DrawRequestEvent;
 pub struct UndoRequestEvent;
 
 /// Request to start a new game. `seed = None` uses a system-time seed.
+/// `mode = None` reuses the current game's `GameMode`.
 #[derive(Event, Debug, Clone, Copy, Default)]
 pub struct NewGameRequestEvent {
     pub seed: Option<u64>,
+    pub mode: Option<GameMode>,
 }
 
 /// Fired by `GamePlugin` after any successful state mutation. Rendering and