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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>