How to detect if the game has ended in the realtime mode

With two bots, status is still `status: in_game`, even after the game is over. Similarly, it looks like `observation.player_result` cannot used for that either, as the list is always empty in realtime mode.

Game returns `ActionResult.YouCantControlThatUnit` when trying to use units after the game is over, however is quite kludgy way to detect if the game is over, partially because it requires executing an action.

Read the realtime-mode protocol definitions and inspect how status, observation.player_result, and ActionResult are represented. Determine what reliable game-ended signal the protocol should expose without requiring an action; done means the behavior is specified and represented consistently for realtime games.

Written by the indexing model from the issue text.