core: add turn-local undo + canUndoActivePlay()

[nodots]

• Push pre-move snapshots in Game.move and executeAndRecalculate\n- Initialize activePlay.undo on roll→moving (post-construction)\n- Add Game.undoLastInActivePlay and Game.canUndoActivePlay\n- Gate switchDice to require empty undo stack and all moves undone\n\nCross-repo change related to nodots-backgammon#217