[Stretch] Undo/replay system

[bh2smith]

Summary

Immutable state history enabling undo (one step back) for solitaire games, and full replay for post-game review.

Tasks

• Each game's state mutations push a snapshot onto a history: GameState[] stack
• undo() pops the last state and re-renders (only available in Golf and Pyramid by default)
• replay() steps through history array at configurable speed (500ms/step)
• UI: 'Undo' button in action area during solitaire games; 'Replay' button on GAME_OVER
• History is session-only (not persisted); cleared on new game

Notes

• Opt-in per game: Golf ✓, Pyramid ✓, Blackjack ✗ (would break betting integrity), Gin ✗ (competitive), Hearts ✗
• Memory: 13 tricks × 4 players = 52 state snapshots per Hearts round — acceptable