Enable viewers to "play back" turns

[slifty]

The game manifest will include the necessary information to re-render each move in the game. Viewers should be able to play back moves and rounds to understand how the map came to be.

[louh]

Yes!

The boardgame.io framework requires that move functions are pure functions. Similar to Redux actions, this allows us to rewind state if necessary. Playback of the event log should be easy to implement.