- Add Reward enum to solitaire_core with CardBack/Background/BonusXp/Badge variants
- Wire rewards into ALL_ACHIEVEMENTS per architecture spec
- evaluate_on_win now applies rewards on first unlock: pushes cosmetic
indices into PlayerProgress, awards BonusXp (with level-up detection),
and marks reward_granted = true so rewards are never double-granted
- Add selected_card_back / selected_background fields to Settings
- Settings panel grows Card Back and Background cycle rows, shown only
when the player has unlocked more than the default (index 0)
- cycle_unlocked() cycles only through earned options
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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